Learners who are limited in their opportunities of listening to examples of the target language tend to form hypothetical rules about the new language on insufficient evidence.Learners need to create new utterances，but they may make errors.Learners'errors are significant in three different ways.（1）First to the teacher，in that they tell him，if he undertakes a systematic analysis how far towards the goal the learner has progressed，and consequently，what remains for him to learn；（2）They provide to the researcher evidence of how language is learned or acquired，what strategies or procedures the learner is employing in his discovery of the language；（3）They are indispensable to the learner himself，because we can regard the making of errors as a device the learner uses in order to learn.The making of errors then is a strategy employed both by children acquiring their mother tongue and by those learning a second language.

The conclusion to be drawn is that errors in learning are significant.They are not，however，entirely caused by differences between the native language of the learner and the language he is learning.There is some value in analyzing the reasons for errors，since this will lead at least to a greater understanding of the difficulties that learners face，and will perhaps assist in the development of pedagogic strategies.The errors will reveal either where the mother tongue does influence learning or where learners are particularly likely to make incorrect generalizations about the target language.It is not necessary desirable that this should be carried out as a purely predictive process.Instead it can be based on the known errors of learners.The errors are still explained in terms of contrast，but contrast on the one hand with the mothertongue and on the other within the target language.The resulting analysis is predictive in the sense that the linguistic behavior of second language learners in the future is expected to resemble closely the behavior of language learners in the past.That is why it is worth analyzing the errors that learners have made.

From the discussions above，it can be concluded that mother tongue as a learner's strategy has played a positive role in the G-T Method.

The major innovation of the American school of contrastive analysis，following the work of Charles Fries and Robert Lado，was to propose that the gaps were generally filled，and some of the correct knowledge was replaced or confused，by the grammar of the learner's first language.Thus，a second language learner's knowledge of a second language was subject to interference from his or her knowledge of the first Language.The implication of this is the clearest if we follow the Saussurean concept whereby language is an abstraction underlying the specific speech behavior of members of a community，or the Chomskyan axiom that a grammar represents the knowledge of the idealized monolingual in a homogeneous speech community.In this sense，interlanguage would represent the knowledge of the idealized speaker in a homogeneous community of second language speakers.In other words，it is a claim that second language speakers share a system that is distinct from the system of native speakers.

It is fair to say that teachers of second or foreign languages are living in very uncertain times.A decade or so ago contrastive analysis was still a fairly new and exciting idea apparently holding great promise for teaching and curriculum construction.The CAH has not proved to be workable，at least not in the strong version in which it was originally expressed.This version can work only for one who is prepared to be quite naive in linguistic matters.In its weak version，however，it has proved to be helpful and undoubtedly will continue to be so as linguistic theory develops.However，the hypothesis probably will have less influence on second language teaching and on course construction in the next decade than it apparently has had in the last decade.One cannot predict whether that diminishing influence will have a good or bad effect on second language teaching.Today CA is only one of many uncertain variables that one must revaluate in second language teaching.No longer does it seem to be as important as it once was.Perhaps，like the Sapir-Whorf Hypothesis，it is too due for a period of quiescence.

From the discussions，it can be seen that in spite of the large number of studies which have been carried out，there is in fact empirical evidence for only an encouragingly small set of causes of IL variation.These causes reduce to（1）linguistic context；（2）a set of social factors so far seemingly limited to interlocutor，topic and social norms；and（3）the differing functions that a given form performs in communicative discourse.Different theories，considered in this chapter seem to highlight one or another of these four factors as causes of interlanguage variation.But it seems clear，that some if not all of these causal factors may interact in any given language-production situation.It seems clear that a learner's focus upon language form，whatever its precise nature，must surely interact with interlocutor，topic and social norm in a fairly complex way.Thus the study of interlanguage is also very important with reference to the G-T Method.