Comparing and Contrasting First and Second Language Acquisition : Implications for Language Teachers

In an attempt to understand and explain first language (L1) acquisition and second language (L2) acquisition scholars have put forward many theories. These theories can aid language teachers to understand language learning and to assist their students in their language learning process. The current paper will first look at the similarities between the L1 and L2 acquisition. Then, the differences will be outlined. In the last part of the paper the implications of these findings for foreign language teachers will be discussed.


Introduction
Various theories are put forward to describe first language (L1) acquisition and second language (L2) acquisition.In order to understand the nature of L1 and L2 language acquisition, various aspects were examined, compared, and contrasted.Results from these comparisons and contrasts have valuable implications for language teachers which can help them to design their syllabuses, teaching processes and classroom activities.These results also enable the language teacher to understand his/her students' learning processes.Many characteristics of L2 acquisition were highlighted by studies conducted on the issue of Interlanguage.Interlanguage theory was developed in the 1970s and 1980s to emphasize the dynamic qualities of language change that make the Interlanguage a unique system.Selinker (1969( , cited in McLaughlin, 1987) ) defines Interlanguage as the interim grammars constructed by second language learners on their way to the target language.Interlanguage is the learner's developing second language knowledge and has some characteristics of the learner's native language, of the second language, and some characteristics which seem to be very general and tend to occur in all or most Interlanguages.It is systematic, dynamic and constantly evolving.
Interlanguages have some common characteristics with L1 acquisition, because both share similar developmental sequences.Some of the characteristics of L2 acquisition show similarities with L1 acquisition, whereas others show differences.

Developmental Sequences
Researchers have carried out numerous studies to understand the nature of first and second language acquisition.These studies have revealed that both first and second language learners follow a pattern of development, which is mainly followed despite exceptions.Rod Ellis (1984) covers the idea of developmental sequences in detail and outlines three developmental stages: the silent period, formulaic speech, and structural and semantic simplification.
Research in natural settings where unplanned language, such as the learner language that results from attempts by learners to express meaning more or less spontaneously, is used to show that both first and second language learners pass through a similar initial stage, the silent period.Children acquiring their first language go through a period of listening to the language they are exposed to.During this period the child tries to discover what language is.In the case of second language acquisition, learners opt for a silent period when immediate production is not required from them.In general, however, many second language learners -especially classroom learners-are urged to speak.The fact that there is a silent period in both first and second language learners (when given the opportunity) is widely accepted.However, there is disagreement on what contribution the silent period has in second language acquisition.While Krashen (1982) argues that it builds competence in the learner via listening, Gibbons (1985, cited in Ellis, 1994) argues that it is a stage of incomprehension.
The second developmental stage is termed formulaic speech.Formulaic speech is defined as expressions which are learnt as unanalysable wholes and employed on particular occasions (Lyons, 1968, cited in Ellis, 1994).Krashen (1982) suggests that these expressions can have the form of routines (whole utterances learned as memorized chunks -e.g.I don't know.),patterns (partially unanalyzed utterances with one or more slots -e.g.Can I have a ____?), and Ellis (1994) suggests that these expressions can consist of entire scripts such as greetings.The literature points out that formulaic speech is not only present in both first and second language acquisition but also present in the speech of adult native speakers.
In the third stage the first and second language learners apply structural and semantic simplifications to their language.Structural simplifications take the form of omitting grammatical functors (e.g.articles, auxiliary verbs) and semantic simplifications take the form of omitting content words (e. g. nouns, verbs).There are two suggested reasons why such simplifications occur.The first reason is that learners may not have yet acquired the necessary linguistic forms.The second reason is that they are unable to access linguistic forms during production.
These three stages show us that L1 and L2 learners go through similar stages of development with the exception that L2 learners are urged to skip the silent period.However, learners do not only show a pattern in developmental sequences, but also in the order in which they acquire certain grammatical morphemes.

Acquisition Order
Researchers have tried to find out if there is an order of acquisition in acquiring grammatical morphemes.The findings are important but contradictory and have implications on first and second language acquisition.Morpheme studies aimed to investigate the acquisition of grammatical functions such as articles or inflectional features such as the plural -s.An important research in this field is that of Roger Brown (1973, cited in McLaughlin, 1987).According to Brown, there is a common -invariant -sequence of acquisition for at least 14 function words in English as a first languagenoun and verb inflections, prepositions, and articles.Findings of these studies pointed out that there is a definite order in the acquisition of morphemes in English first language learners.Other morpheme studies were carried out on various functors suggesting that an order of acquisition does exist.Lightbown and Spada (2006) review studies which have proposed that the acquisition of question words (what, where, who, why, when, and how), show a great similarity in first and second language acquisition.Based on the morpheme studies in L2 acquisition, Krashen (1982) put forward the Natural Order Hypothesis which he developed to account for second language acquisition.He claimed that we acquire the rules of language in a predictable order.This acquisition order is not determined by simplicity or the order of rules taught in the class.
Thus far it seems as if L1 acquisition and L2 acquisition follow similar routes, however, other morpheme studies have shown that not all first language learners follow the order of acquisition predicted.There appears to be inter-learner variation in the order of acquisition.Wells (1986b, in Ellis, 1994) proposes inter-learner variables affecting the order of acquisition as sex, intelligence, social background, rate of learning, and experience of linguistic interaction.Furthermore, McLaughlin (1987) claims that evidence from research shows that the learner's first language has an effect on acquisitional sequences which either slows their development or modifies it.He adds that, considerable individual variation in how learners acquire a second language, such as different learning, performance, and communication strategies, obscure the acquisitional sequences for certain constructions.Therefore, McLaughlin (1987) argues that "Krashen's claim that an invariant natural order is always found is simply not true" (p.33).
The above arguments show that there seems to exist an order of acquisition in both first and second language acquisition.Hence, one should be careful not to claim for an invariant order of acquisition but for a more flexible order of acquisition and be aware of the variations affecting this order.

Linguistic Universals and Markedness
There are two approaches to linguistic universals.The first approach was put forward by Greenberg (1966, in Ellis 1994) and termed typological universals.Typological universals are based on cross-linguistic comparisons on a wide range of languages drawn from different language families to discover which features they have in common (e.g.all languages have nouns, verbs etc.).The second approach is the generative school represented by Chomsky.The aim is to study individual languages in great depth in order to identify the principles of grammar which underlie and govern specific rules.This approach was later termed as Universal Grammar (Ellis, 1994).
The most relevant aspect of both approaches that relates to L1 and L2 acquisition is that some features in a language are marked and some are unmarked.According to typological universals, unmarked features are those that are universal or present in most languages and which the learners tend to transfer.Marked rules are language specific features which the learner resists transferring.According to Universal Grammar, core rules, such as word order, are innate and can be arrived at through the application of general, abstract principles of language structure.Peripheral rules are rules that are not governed by universal principles.Peripheral elements are those that are derived from the history of the language, that have been borrowed from other languages, or that have arisen accidentally.These elements are marked.Peripheral aspects are more difficult to learn (Ellis, 1994;McLaughlin, 1987).
Even though neither of these approaches aimed at explaining first or second language acquisition, the results of both are applicable.The findings show that unmarked features are learned earlier and easier than marked rules in both the first and the second language while unmarked forms require more time and effort by the learner.

Input
Input is defined as "language which a learner hears or receives and from which he or she can learn" (Richards et al., 1989, p. 143) and its importance is widely accepted.Behaviorist views hold that there is a direct relationship between input and output.In order to obtain favorable habits the language learner must be given feedback, which constitutes the input.Interactionist views of language acquisition also hold that verbal interaction, or input, is crucial for language acquisition.
Stephen Krashen (1982) has put forward the Input Hypothesis which reveals the importance he places on input.He argues that the learner needs to receive comprehensible input to acquire language.Information about the grammar is automatically available when the input is understood.Krashen argues that the input a first language learner receives is simple and comprehensible at the beginning and is getting slightly more complicated.With this argument, he supports his next argument that input should be slightly above the level of the language learner (i+1).Only in doing so can the second language learner move forward.He argues that the second language learner should be exposed to the target language as much as possible and that the lack of comprehensible input will cause the language learner to be held up in his development (Ellis, 1994;McLaughlin, 1987).
The Interactionist Approach to first language acquisition holds that one to one interaction gives the child access to language which is adjusted to his or her level of comprehension, therefore, interaction is seen as crucial and impersonal sources of language (such as TV and radio) are seen as insufficient.Consequently, verbal interaction is seen to be crucial for language leaning since it helps to make the facts of the second language salient to the learner.Similarly, intersectional modifications which take place in the conversations between native and non-native speakers are seen as necessary to make input comprehensible for the second language learner (Lightbown & Spada, 2006;Ellis, 1994).
There is, however, a contradicting view to the importance of input in first and second language acquisition.Chomsky (see Ellis, 1994;McLaughlin, 1991) argues that input is essential but that input alone cannot explain first language acquisition because it contains ungrammaticalities and disfluencies which make it an inadequate source of information for language acquisition.Children would not be able to distinguish what is grammatical and ungrammatical based on such input.Furthermore, input underdetermines linguistic competence.He argues that input alone does not supply learners with all the information they need to discover rules of the L1.Therefore, he points out that the child must be equipped with knowledge that enables the learners to overcome the deficiencies of the input.Later, Universal Grammar researchers have drawn implications to second language acquisition from these arguments.It is believed that the same arguments for the inadequacy of input in first language acquisition also account for second language acquisition.Consequently, when learning a first language, learners must rely on the knowledge they are equipped with; and when learning a second language, learners must rely on the L1.
These arguments show us that both input and the knowledge that the child is equipped with are important and should interact for learning and development to take place.Therefore, one should not be favored over the other.

Behavioristic Views of Language Acquisition
The similarity between L1 and L2 acquisition is seen in the Behavioristic Approach originally which tries to explain learning in general.The famous psychologist Pavlov tried to explain learning in terms of conditioning and habit formation.Following Pavlov, B. F. Skinner tried to explain language learning in terms of operant conditioning.This view sees language as a behavior to be taught.A small part of the foreign language acts as a stimulus to which the learner responds (e.g. by repetition).When the learner is 100 % successful, the teacher reinforces by praise or approval.Consequently, the likelihood of the behavior is increased.However, if the learner responds inappropriately then the behavior is punished and the likelihood of this behavior to occur is decreased (Brown, 1994).In other words, children imitate a piece of language they hear and if they receive positive reinforcement they continue to imitate and practice that piece of language which then turns into a 'habit' (Williams & Burden, 1997).
Similarly, basing on the Behavioristic Approach it is assumed that a person learning a second language starts off with the habits associated with the first language.These habits interfere with those needed for second language speech and new habits of language are formed.Errors produced by the second language learner are seen as first language habits interfering with second language habits.This approach advises the immediate treatment of learner errors (Lightbown & Spada, 2006).Some regular and routine aspects of language might be learned through stimulus/response but this does not seem to account for the more grammatical structures of the language.The Behavioristic Approach holds that language acquisition is environmentally determined, that the environment provides the language learner with language, which acts as a stimulus, to which the language learner responds.However, L1 and L2 learners form and repeat sentences they have not heard of before.Therefore, this approach fails to account for the creative language use of L1 and L2 learners.

Zone of Proximal Development (ZPD)
The Russian psychologist L. S. Vygotsky has made a social emphasis on education in general and language education in particular.Vygotsky (1982, cited in Daniels, 1996, p. 171-172) explains the ZPD as follows: "The child is able to copy a series of actions which surpass his or her own capacities, but only within limits.By means of copying, the child is able to perform much better when together with and guided by adults than when left alone, and can do so with understanding and independently.The difference between the level of solved tasks that can be performed with adult guidance and help and the level of independently solved tasks is the zone of proximal development."(p.117) When children come across a problem they cannot solve themselves they turn to others for help.Thus, collaboration with another person is important for a child to learn.Otherwise, development would not be possible.Learning collaboratively with others precedes and shapes development.A good example for this process is said to be the development of literacy (Gallaway & Richards, 1994;Lantolf & Thorne, 2007).
Vygotsky asserts that through using language children take part in the intellectual life of the community.In order to negotiate meaning, collaboration between the child and the members of the community is required.Considering language education, instruction creates the zone of proximal development, stimulating a series of inner developmental processes (Daniels, 1996;Lantolf & Thorne, 2007).According to the ZPD, assistant performance and collaboration are crucial for learning and development.The teacher's assistance and students' collaboration with their teacher and their peers is inevitable for L2 development.The teacher's most important classroom work "is to provide for the social interaction within the community of learners such that the learners may move from what they know to what they don't yet know" (Hawkins, 2001, p. 375).
The ZPD also asserts that "what one can do today with assistance is indicative of what one will be able to do independently in the future" (Lantolf & Thorne, 2007, p. 210).Thus, development achieved and development potential are equally emphasized.The ZPD concept can aid educators to understand aspects of students emerging capacities that are in early stages of maturation (Lantolf & Thorne, 2006).Krashen (1982) claims that there are two ways for an adult to approach a second language:

The Acquisition/Learning Hypothesis
"adults can (1) 'acquire,' which is the way children 'get' their first language, subconsciously, through informal, implicit learning.Once you have acquired something you're not always aware you have done it.It just feels natural; it feels as if it has always been there.Quite distinct from acquisition is (2) conscious learning.This is knowing about language, explicit, formal linguistic knowledge of the language."(p.17) Krashen continues to argue that learning does not turn into acquisition.He obviously sees first language acquisition and second language acquisition as two different phenomena.Yet, he suggests that acquisition may occur in the classroom when communication is emphasized through dialogues, role playing, and other meaningful interaction.
As a language teacher, one should be careful when evaluating the claims related to acquisition and learning.Through focused input and focused practice learning may turn into acquisition.

The Critical Period Hypothesis
The Critical Period Hypothesis holds that there is "a biologically determined period of life when language can be acquired more easily and beyond which time language is increasingly difficult to acquire" (Brown 1994, p. 52).This hypothesis is based on the ideas of the psychologist Eric Lenneberg.His argument was that various capacities mature according to a fairly fixed schedule during which language emerges in children when anatomical, physiological, motor, neural, and cognitive development allow it to emerge.He added that there is a critical, biologically determined period of language acquisition between the ages of 2 and 12 (McLaughlin, 1987).Originally the notion of critical period was connected only to first language acquisition but later it was applied to second language acquisition as well.Consequently, it is argued that a critical period for second language acquisition is due until puberty.
In order to explain the validity of the critical period in second language acquisition neurological, psychomotor, and cognitive arguments were examined (Brown, 1994).These have mostly tried to explain why adult language learners are not able to reach full competence and native like pronunciation in the second language.
Neurological Considerations: There is an attempt to explain the difference between first and second language acquisition through lateralization in the brain.Steinberg (1997) explains lateralization as follows, "the brain assigns, as it were, certain structures and functions to certain hemispheres of the brain.Language, logical and analytical operations, and higher mathematics, for example, generally occur in the left hemisphere of the brain, while the right hemisphere is superior at recognizing emotions, recognizing faces and taking in the structures of things globally without analysis.This separation of structure and function in the hemispheres is technically referred to as lateralization".(p.179) Thomas Scovel (1969( , in Brown, 1994) ) put forward that there is a relationship between lateralization and second language acquisition.Scovel suggests that the plasticity of the brain before puberty enables first and second language acquisition to take place easily.After puberty, the brain looses its plasticity and lateralization is accomplished.He argues that lateralization makes it difficult for people to be able ever again to easily acquire fluent control of the second language or native-like pronunciation.
There is a counter argument related to the cognitive development of the brain.Cognitively, this lateralization enables the person to reach the capability of abstraction, of formal thinking, and of direct perception which start from puberty on.This shows that adults posses superior cognitive capacity due to left hemisphere dominance.Then, the following question arises: How come that adults who have a cognitive superiority are not able to learn a second language successfully?Researchers are still trying to find an answer to this question.A tentative answer to this question is that the dominance of the left hemisphere leads the adult to tend to overanalyze and to be too intellectually centered on the task of second language learning (Brown, 1994).Again, there are adults who are able to learn a second language successfully, but factors like affective variables seem to play an important role in such cases.
Psychomotor Consideration: These considerations try to explain the reason why adult second language learners cannot obtain native-like pronunciation in the second language.Starting from birth, speech muscles gradually develop until after the age of 5.Then, until puberty the speech muscles maintain their flexibility.Scientists argue that the flexibility of children's speech muscles is the reason for why they can easily acquire native-like pronunciation both in the first and in the second language.The decline of the flexibility in the speech muscles, however, prevents adult second language learners to reach native-like pronunciation in the second language (Brown, 1994).
Affective Considerations: Although the affective domain includes many factors such as inhibition, attitudes, anxiety, and motivation, this paper will examine only the first two.While anxiety and motivation are mainly related to adult second language learning, child first language learners have not developed or are just in the process of developing such affective factors.
While inhibitions pose no difficulty for children acquiring their first or second language, they propose to be intervening in adult second language acquisition.Inhibitions can be defined as ego boundaries the person builds in order to protect his or her ego.As the child matures it develops a sense of self-identity and towards puberty it acquires the feeling to protect this self-identity and develop inhibitions which are heightened during puberty.Alexander Guiora (cited in Brown, 1994) proposed the idea of the language ego to account for the identity a person develops in reference to the language he/she speaks.Through puberty the child's ego is flexible and dynamic but as the child reaches puberty the language ego becomes protective due to physical, cognitive, and emotional changes at this stage.The language ego tries to protect the ego of the young adult by clinging to the security of the native language.Acquiring a second language means also acquiring a new language ego which can be very difficult for adults who have built up inhibitions to protect their ego.Mistakes can be seen as threats to one's ego.With the fear to make mistakes the adult language learner can resist to speak in the classroom.
A second affective factor, which is formed by the cognitive development of a person, that can make second language acquisition difficult for an adult is attitude.Young children are not cognitively enough developed to possess attitudes towards races, cultures, ethnic groups, and languages.As the child reaches school age, attitudes are acquired.It is agreed that negative attitudes towards the target language, target language speakers, the target language culture, and the social value of learning a second language can impede language learning while positive attitudes can enhance learning (Ellis, 1994;Brown, 1994).
Stephen Krashen has developed The Affective Filter Hypothesis to account for the effects of affective variables on second language acquisition.He argues that affective variables can act as a mental block, also termed affective filter, and prevent comprehensible input to be absorbed.When the learner is unmotivated and lacks confidence the affective filter goes up.When the learner is not anxious and wants to be a member of the group speaking the target language the filter goes down.He adds that children are at an advantage when learning a first or second language because their affective filter is low while adults are likely to have a higher affective filter due to events that occurred in adolescence (Krashen, 1982;McLaughlin, 1987).
The critical period shows concrete differences between L1 and L2 acquisition because it is based on the internal factors of the learner.The arguments of the critical period are mainly based on pronunciation, neglecting grammatical and semantic competence.

Fossilization
Fossilization is used to label the process by which non-target norms become fixed in Interlanguage.The possible causes for fossilization are suggested to be age (learners' brains loose plasticity at a critical age, therefore, certain linguistic features cannot be mastered), lack of desire to articulate (learners' make no effort to adopt target language norms because of various social and psychological factors), communicative pressure (the learner is pressured to communicate ideas above his/her linguistic competence), lack of learning opportunity, and the nature of the feedback on learners' use of L2 (positive cognitive feedback leads to fossilization while negative feedback helps avoid fossilization) (Ellis, 1994;McLaughlin, 1987).
Based on the factors related to fossilization it can easily be inferred that fossilization is unique to L2 acquisition.It is hardly possible to see a child acquiring his/her first language to fossilize certain forms of language.Ellis (1994) differentiates between two social contexts in second language learning and outlines them as follows:

a. Natural Contexts
Second language learning in majority language contexts: the target language serves as the native language and the language learner is a member of an ethnic minority group (e.g.Turkish workers in Germany).
Second language learning in official language contexts: the second language functions as an official language (e.g.English in Nigeria).
Second language learning in international contexts: the second language is used for interpersonal communication in countries where it is neither learnt as a mother tongue nor used as an official language (e.g. in arts, science, academic, etc.)

b. Educational Contexts
Segregation: the second language is taught to learners in a separate context from the native speakers of the target language.
Mother tongue maintenance: learners of minority groups are either given classes in their mother tongue or they are educated through the medium of their mother tongue.Submersion: right from the beginning L2 learners are taught with native speakers.
The language classroom: the target language is taught as a subject only and is not commonly used as a medium of communication outside the classroom.
The difference of the contexts of first and second language acquisition play an important role in the acquisition process.While it is possible to learn a second language in various contexts, first language acquisition takes place only in a natural context and in the social group the child is growing up and where the child gets L1 input only.The different contexts for second language acquisition can also lead to variations in second language proficiency due to affective factors.Schuman (1986, cited in McLaughlin, 1987;Ellis, 1994) has put forward the Acculturation Theory to account for second language acquisition development in natural settings.He defines acculturation as the process of becoming adapted to a new culture and his claim is that contact with the target language and culture is crucial.The process of acculturation requires both social and psychological adaptation.Learning the appropriate linguistic habits to function within the target language group is one part of this process.Acculturation is determined by the degree of social and psychological 'distance' between the learner and the target-language culture.According to this hypothesis, the greater contact with L2 speakers and culture takes place the more acquisition occurs.
Another social factor that leads to a difference between first and second language acquisition is that of the learner's choice of target language variety.SLA assumes that learners are targeted at the standard dialect of the L2.Beebe (1985, in Ellis, 1994) observed some deviations in L2 learners' from Standard English.She suggests that these may not be errors but a reflection of a dialect which the learner has targeted (e.g.Black English).The choice of the reference group is determined by the social context and the learner's attitude to that variety of language.In settings where the L2 is an official language (such as in India), the reference group may be educated users of the L2 in the learner's own country rather than a native speaker.
It is important to note here that in first language acquisition one has no chance to make such a conscious choice.The environment and social group a person is born into automatically determines the language variety to be acquired.
Therefore, deviations from the standard language are not seen as a failure to acquire the language.However, such deviations may wrongly be attributed to failure if present in the second language.

Implications for Language Teachers
To understand the nature of L1 acquisition, researchers have tried to explain how children progress from "no language" to their mother tongue.In L2 acquisition, however, the process is more complicated as learners already have knowledge of their L1.The Interlanguage Theory plays a crucial role in arriving at findings on how L2 learners move from their mother tongue towards the target language.This means that we cannot talk about the Interlanguage of a child but that we can talk about the Interlanguage of the L2 learner.
Language learning to take place depends on various factors, which means that the language teacher has to account for these factors as much as possible.However, none of the theories or factors mentioned in this paper is on its own explanatory enough to account for the complex process of language learning.Every finding or explanation should be considered in interaction with the others.This means that a language teacher cannot base his/her teaching solely on any single theory or claim within the framework of L2 or L1 acquisition.
The above similarities and differences between first and second language acquisition provide the language teachers with information to aid them in their profession.This information can help the teacher in designing classroom activities, designing the syllabus, choosing an appropriate method, understanding the learning processes of his/her students, and guiding his/her students in the language learning process.
The first discussion in terms of the similarities and differences between L2 and L1 acquisition was related to developmental sequences which plays an important role in the cognitive development of learners.Knowing that in L1 acquisition learners have the right to keep silent and process the input would be quite beneficial under ideal teaching situations.Even though this silent period promotes language processing in L1 acquisition, it is quite hard, even impossible, to apply it in L2 acquisition.The teaching conditions and the grading legislations may force the teacher to ask students for immature production.Knowing the need for such a period but not being able to allow for it should at least make the teacher understand erroneous production, inhibited students, or high anxiety in the classroom.Even though the idea of silent period may not be applicable directly into teaching, it gives an idea of why some students resist or avoid to produce the language taught.Formulaic speech, the second stage of developmental sequences, is said to be present in both first and second language acquisition and also present in the speech of adult native speakers.Thus, language teachers might consider teaching their students samples of some useful and frequently used phrases.Students can then refer to these phrases in situations that require immediate communication.Finally, in the last step of developmental sequences learners apply structural and semantic simplifications to their language.Knowing this can help language teachers understand erroneous or imperfect language production of certain language items such as omitting language functors or omitting content words.
The second phenomena is acquisition order of language learning (both L1 and L2), and can have a great impact on syllabus design.As Krashen (1982) put forward, we acquire the rules of language in a predictable order.Knowing which structures are learned prior to others helps in ordering the content of the syllabus.Similarly, studies on marked and unmarked structures correspond to the acquisition order.Designing the syllabus by taking these findings into account takes the burden of trying to figure out which structures to teach first and which ones later.
The notion of markedness also has implications for language teachers.It is asserted that marked features are learned earlier and easier than marked rules in both the first and the second language.On the other hand, unmarked forms require more time and effort by the learner and are more difficult to learn (Ellis 1994;McLaughlin, 1987).Considering markedness, language teachers could find out the unmarked features of the target language and plan their lessons so that they spend more time on unmarked features.Furthermore, the idea of markedness could help teachers understand why their students fail to learn or have difficulty in learning certain features of the target language.
The issue of input has an explanatory effect both in L1 and in L2 acquisition, which means that it has direct implications for the language teacher.Language teachers are the main source of input in the classroom.A teacher wishing to provide comprehensible input will have to modify his/her language according to the level of students and speak at such a speed that the students can follow.The teacher can use lots of activities requiring the students to interact with each other or with the teacher.Pair work, information gap activities, and classroom discussions are a good source for input.However, overwhelming students with input that is quite above their language capacity might result in lack of self-confidence and resistance to learn the language.So, using input to promote language learning is beneficial as long as it is aimed at the level of the students.
Even though the Behavioristic approach lacks to explain the creative aspect of language production, it helps to understand how in teaching/learning, stimulus/response helps to master both grammatical patterns and phonological patterns.To make use of this knowledge at the right time in the process of teaching depends on whether the teacher has been able to identify when stimulus/ response can be used for the benefit of the learning.However, an important point here which is to be kept in mind is the age and level of the students in mind.While younger learners might find such an education enjoyable, older learners might get easily bored.In addition, learning language habits might be useful for students with lower levels of proficiency; students with a higher level of proficiency may not benefit the same amount.
The final issue related to the similarities of L1 and L2 acquisition is the ZPD.The importance of assistance and collaboration has useful implications for language teachers.Language teachers should try to assist their students as much as possible by providing them with language necessary to pass to the next level of language competence.The role of the teacher is to direct action within school activity in a manner appropriate to the learner's level of development, the cultural and social environment (Daniels, 1996).It can also be suggested that teachers promote teacher-student interaction or peer-interaction.As Hawkins (2001) states, "It is via this kind of interaction that knowledge very gradually gets built" (p.374).This is possible through the use of collaborative activities such as pair work or group work where students are required to negotiate meaning.Furthermore, teachers could benefit from the ZPD to understand aspects of students' emerging capacities.That is, language tests should be viewed as both indicators of students' achieved abilities and also students' future capabilities.
When we come to the differences attributed to L1 and L2 acquisition, the starting point should be terms themselves, "acquisition" and "learning".Although it is argued that learning and acquisition are quite distinct processes, a language teacher should consider the possibility that extensive practice in the classroom can lead to acquisition.However, it should be kept in mind that not everything taught becomes acquired.So, expectations regarding the quality of learning should be set realistically.
The Critical Period hypothesis is one of the key differences leading to variations in L1 and L2 acquisition.It is widely accepted that children are better in pronunciation, whereas adults are faster and better learning in rules and pragmatics.Knowing this may guide a teacher who is teaching adults towards practicing pronunciation, if this is one of the objectives in learning the language on the side of the learner.Another important point related to the critical period regards affective factors.While it does not cause a problem in L1 acquisition, the learners of L2 are faced with inhibition and attitudes.The affective states of our students are very important since these are the major factors intervening in language learning.Adult or young adult language learners need to be relaxed and comfortable to create positive attitudes to the language and the language learning process.In addition, teachers need to free their students from inhibitions so that students can freely interact and use the language.This can only be possible if they build up trust and understanding between themselves and their students.More positive than negative feedback, more praise than criticism might be the first step.
The issue of fossilization is only attributable to L2 acquisition.While all L1 learners reach full competence in the target language, some forms in the target language of the L2 learners might be fossilized.Teachers can prevent fossilization by correcting repeated errors of their students or they can practice problematic language more than non-problematic language.One should be aware that once fossilization takes place, it is very difficult to get rid of.Therefore, teachers should act with caution and help their students to prevent fossilization.
The last factor to be mentioned regards social issues.It was previously stated that second language learners may choose to learn a language variety other than the standard form depending on the speech community they are taking as a reference.Such is the case in natural settings and not in classroom settings.Therefore, it is the teacher's (or the teaching institution's) responsibility to decide on which variety of the target language to take as the norm.It is important to make students aware of the different varieties of the target language, but in terms of teaching, there should be consistency.

Conclusion
L1 and L2 acquisition are quite complicated processes.To understand these processes will enable the language teacher to be more sensitive to the factors involved.While L1 and L2 acquisition reveal some similarities, they also show differences.The teacher should understand that the phenomena in L1 and L2 acquisition are interacting, none of them being solely explanatory.So, teachers should not base their teaching on just a single claim or factor involved in language acquisition.They should rather understand, analyze, synthesize and even criticize before trying to implement any of the suggestions made for teaching.
It is also important to note that research as tried to make a distinction between "learning" and "acquisition".Especially in L2 education, the terms "learning" and "acquisition" are very often used interchangeably.
The arguments considering L1 and L2 acquisition are inconclusive and that's why many studies were conducted to explain the nature of L1 and L2 acquisition.L1 and L2 acquisition are affected by many variables.Thus, the student's profile itself is an important determiner at the decision making phase of language teaching.Finally, language teachers should combine their theoretical knowledge with their teaching situation.