Whenever the speaker of one language learns to speak another language the acquisition takes place gradually, not all at once, and is characterized by successive changes in the person’s knowledge of the second language and his or her ability to use it. The development proceeds from zero mastery of the new language (except insofar as universal human language capabilities and mastery of the first language constitute knowledge of the new language) toward full mastery, although en route there may be detours and retracing of steps and full mastery may not be achieved. It is reasonable to assume that at any point in the development the speaker has a ‘grammar’, a linguistic system that is an incomplete and in part incorrect version of the grammar of the target language, and his ‘errors’ in comprehension and production of the target language may be used, along with his ‘correct’ behaviour, as evidence for the current stage of his learner’s grammar. Ferguson 1963 [Ferguson, C. A. 1963. “Linguistic theory and language learning”, Monograph Series on Languages and Linguistics 16, pp. 115-124] pointed to the value of studying such partial grammars, to which Nemser [Nemser, W. 1971. “Approximative systems of foreign language learners”, International Review of Applied Linguistics in Language Learning, 9, pp. 115-123] gave the name of ‘approximative systems’ and which Selinker and colleagues have called ‘interlanguage’ [Selinker, L. 1972. “Interlanguage”, International Review of Applied Linguistics 10: 209-32].
- Ferguson-DeBose (1977), a pag.107