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Interlanguage is the learner's current version of the language they are learning. Interlanguage changes all the time but can become fossilised language when the learners do not have the opportunity to improve.
A learner who has lived in an English-speaking environment for many years but not attended classes or studied for some time has problems with reported speech and forming tag questions. His interlanguage has become fossilised.
In the classroom
Interlanguage is often heavily influenced by L1 and interference from this may make it seem perfectly logical to the learner, although it is incorrect. It is important for teachers to understand this and also to see interlanguage as a series of learning steps.