Interlanguage is the implicit linguistic system adult L2 learners construct, intermediate between native and target languages. Interlanguage is systematic (not random error), develops through stages, and provides evidence that adult learners apply unconscious linguistic principles rather than conscious rules.
Analyze interlanguage speech and writing for systematic patterns and developmental stages; compare learner errors across L1 backgrounds to distinguish transfer from universal acquisition strategies.
Interlanguage errors are not mere performance slips; they reflect learner competence and underlying hypotheses about the target system structure.
From your study of second language acquisition and parameter setting, you know that learners don't merely memorize vocabulary and rules — they construct implicit mental grammars, and acquiring a language involves setting abstract parameters that differ between languages. The concept of interlanguage applies this framework directly to adult L2 learning: at any given stage of acquisition, the learner has a specific mental grammar that is neither their native language (L1) nor the target language (L2) — it is a distinct, systematic linguistic system in its own right.
The defining claim of interlanguage theory is that learner errors are systematic, not random. A Spanish speaker learning English who consistently places adverbs between the verb and object ("He watches always the news") is not making careless mistakes — they are applying a consistent structural principle derived from Spanish, where this order is grammatical. This is L1 transfer: the learner's existing grammar provides one source of hypotheses about the target language. But transfer is not the whole story. Learners of many different L1 backgrounds make *similar* errors when acquiring the same L2 feature — suggesting that universal acquisition strategies, not just L1 interference, shape the developmental path. The sequence for acquiring English negation (from "no + verb" to "don't + verb" to full auxiliary negation) appears across learners regardless of their mother tongue.
The concept of fossilization — the stabilization of certain interlanguage features that stop progressing toward the target — raises deep questions about the critical period hypothesis you encountered in parameter-setting acquisition. Adult learners rarely achieve fully native-like command of every feature even after years of immersion and instruction. Some phonological and morphological features appear to resist acquisition past a certain age, suggesting that adult language learning may rely on different cognitive mechanisms than child acquisition — possibly with reduced access to the parameter-resetting processes that allow children to acquire any human language from near-zero. The interlanguage in such cases is not a failed version of the target; it is a stable grammar that the learner has stopped revising.
The practical consequence of interlanguage theory is that learner errors become diagnostic rather than merely wrong. Analyzing which errors are systematic, which reflect L1 transfer, which reflect universal developmental stages, and which are fossilized allows researchers and teachers to understand where learners actually are in their acquisition trajectory — not just how far they are from a target. This reframing — treating the learner's grammar as real and worthy of analysis, not simply as deficient — was one of the significant conceptual shifts in twentieth-century applied linguistics.