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It is assumed that while a learner is learning a second language，he is building his own language system of abstract linguistic rules which serve as a base for his comprehension and production of the L2.This particular language system constructed by the learner is different from both his mother tongue and the target language.The American linguist，Larry Selinker（1972）introduced the term“Interlanguage”to refer to the system of a learner's language as it moves gradually from the L1 toward the target L2.The development of IL is considered a creative process，which is driven by inner forces in interaction with environmental factors and which is influenced both by L1 and by the input from the target language.A learner's interlanguage is a unique language system.The interlanguage is also called learner language since it is constructed by the learner.
There have been some slightly different conceptualizations of learner language.For example，Nemser（1971）referred to it as“approximative system”，while Corder（1981） termed it“transitional competence”.However，the notion of“interlanguage”seemed to prevail and was used in the literature on second language acquisition in the 1990s.
The term interlanguage was defined as the separate linguistic system which could be evidenced when adult L2 learners attempt to express meaning in a language they are in the process of learning.The linguistic system includes not only phonology，morphology，and syntax，but also the lexical，pragmatic，and discourse levels of the interlanguage.Interlanguage is usually thought of as characteristic only of adult second language learners，who have passed puberty and thus cannot be expected to employ the language acquisition device（LAD）—the innate language learning structure that was instrumental in the acquisition of their native language.Children acquiring second languages are thought to have the ability to re-engage the LAD.Second language learners who begin their study of the L2 after puberty，however，do not succeed in developing a linguistic system as children do when acquiring that language natively.This observation led Selinker（1972） to hypothesize that adults use a latent psychological structure，instead of an LAD，to acquire second languages.Selinker claimed that interlanguage depends on five central processes that are part of the“latent psychological structure”：
（1）Native language transfer.Although L1 transfer is not the only process involved，ample research evidence shows that it does play an important role in shaping learners’interlanguage systems.Selinker suggested that the way in which this happens is that learners make“interlingual identifications”in approaching the task of learning a second language：they perceive certain units as the same in their NL，IL and TL.For example，they may perceive NL“开”as exactly the same as TL“open”as in the expression“open the door”，and develop an interlanguage in which“open”can be used（erroneously in terms of the TL） in expressions like“ open the light”，“ open the meeting”，and so on.
（2） Overgeneralization of target language rules.This psycholinguistic process is also widely observed in child language acquisition.The learner shows evidence of having mastered a general rule，but does not yet know all the exceptions to that rule.A well-known example is that the learner may use the past tense marker -ed for all verbs，regular and irregular alike.They may say“I walked to school”，and“He laughed ”；they may also say“I goed to school”，and“He drinked beer”.
（3） Transfer of training.Transfer of training occurs when L2 learners apply rules learned from teachers or textbooks.Sometimes such learning is successful；that is，the resulting interlanguage rule is the same as the target language rule.But sometimes errors occur.For example，a lesson plan or textbook that describes the past perfect tense as the“past past”can lead the learner to erroneously use the past perfect for the distant past.So，for example，they may say“ I had learned English in the 1980s ”.Such errors are also called“induced errors”.
（4） Strategies of communication.Strategies of communication are often used by L2 learners to resolve communication problems.When the learner attempts to communicate meaning and feels that the linguistic item needed is not available，he can resort to a variety of strategies of communication in getting the meaning across.We discussed eight communication strategies in chapter one，which are treated as the internal factors affecting L2 learning.In communicating meaning，L2 learners create linguistic forms and patterns which are different from idiomatic target language expressions.These linguistic forms and patterns used in such attempts may become more or less permanent parts of the learner's interlanguage.
（5）Strategies of L2 learning.L2 learners use learning strategies in a conscious attempt to master the target language.One such strategy is simplification.For example，the learner“simplifies” English so that all verbs may occur in the present continuous，yielding sentences like“ I'm hearing him ”.Another strategy is learners’conscious comparison of what they produce in the IL with the NL，setting up interlingual identifications.Other examples of learning strategies are the use of mnemonics to remember target vocabulary，the memorizing of verb declensions or textbook dialogues，the use of flash cards，and so on.Research evidence showed that all five of these psycholinguistic processes could affect the construction of interlanguages.
Since its first detailed proposal in 1972，the interlanguage hypothesis has experienced some modifications and expansions.The first expansion is the inclusion of children's second language acquisition.The original interlanguage hypothesis was restricted to apply only to adults acquiring second languages.However，evidence emerged that children in language immersion programs also produced interlanguages，resulting from L1 transfer.There appear to be sociolinguistic reasons for this phenomenon.The children receive native-speaker input only from their teacher，and give one another non-native input.Therefore，they did not have enough opportunity or incentive to produce what Swain calls“comprehensible output”．To the extent that these children produce interlanguages in these contexts，there is some question whether they are using LADs to internalize the target language or whether they are using those psycholinguistic processes described as more characteristic of adults learning second languages.More research is needed to find out how they differ from adult learners.
A second expansion of the interlanguage hypothesis has occurred in response to the growing interest in the influence of universal grammar（UG） on the development of interlanguage.The crucial question was this：UG is assumed to be central to the development of natural languages，but is interlanguage a natural language？Two positions are taken in response to this question.The first position，as Selinker's initial hypothesis takes，is that it is not，at least as the notion“natural language”has been defined in linguistics.This early position argues that natural languages are produced by LADs；language universals exist in human languages by virtue of the way in which the language acquisition device is structured；but interlanguages fossilize and evidence native language transfer；interlanguages therefore are a product of latent psychological structures，not LADs；so，interlanguages do not have to obey language universals.The opposing position holds that interlanguages are natural languages，and have to obey language universals.Central to this position is the view that interlanguages are products of the same language acquisition device that produces native languages.In this view，interlanguages fossilize because of complex changes in cases where parameters have already been set for one language and a second language must be learned.The debate on this issue is still going on.
A third modification has been a growing emphasis on the way in which interlanguage development seems to vary in different social context，or discourse domains.Increasing evidence seems to show that learners can produce more fluent，grammatical，and transferfree interlanguage in some social contexts than in others.For example，international teaching assistants may be more fluent and grammatical in lecturing on their academic field than when talking about an everyday topic like favorite foods or bicycling.Key processes such as fossilization may be more prominent for a given learner in one context than in another.
A fourth issue centers on the phenomenon of fossilization itself and whether it is inevitable.It was argued by Selinker that no adult learner can hope to speak a second language in such a way that he or she is indistinguishable from the native speakers of that language.There are inevitable forces leading to the cessation of learning.There are neurolinguistic reasons for this inevitability.There is a very common case where an adult learner's phonological system may fossilize，but the morphology，syntax，and lexicon may not，continuing to develop until reaching full identity with the target language.Scovel（1988）， like Selinker， argued that the causes of phonological fossilization are neurolinguistic in nature and related to the process of cerebral lateralization，which is completed at puberty.Those who claim that fossilization is caused by sociolinguistic forces argued that fossilization is not an inevitable process.Such researchers suggest that if learners can identify with the target social group，or if the need is great enough，they will be able to continue learning the second language until their production and perception is indistinguishable from that of native speakers.It should be noted that this issue also is far from settled since it relates to matters of human potential rather than human's actual behavior.
Finally，research on interlanguage has expanded far beyond its original focus on phonology，morphology，syntax，and lexis，to include the sociolinguistic component of communicative competence.Research on interlanguage includes comparative work on the way in which learners execute speech acts across three linguistic systems.For example，Cohen and Olshtain（1981） have studied the way learners attempt to apologize，using their interlanguage，in target language social contexts，and compared this to the way native speakers apologize in the same contexts.
In short，the interlanguage hypothesis provided the initial spark that ignited a field of research on SLA，and it continues to provide what some feel to be the most productive framework for research.