The version of the target language spoken by a learner at any given time during the period of learning . A learner’s interlanguage will change and develop as they become more proficient. Some aspects of it may fossilize as their proficiency develops.
"Learners’ interlanguage can develop quickly if they get enough exposure – you see the way they use different grammatical structures with more precision, the range and appropriateness of vocabulary use and the clarity of their pronunciation really changing fast."
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Skehan, P. (1994.) Second language acquisition strategies, interlanguage development and task-based learning, in Bygate, M., A. Tonkyn, and E. Williams, Grammar and The Language Teacher. Hemel Hempstead: Prentice Hall.