Function – to – form mapping
Another functional approach which has been applied to the description and analysis of interlanguage emphasizes function-to-form mapping in the acquisitional sequence. A basic concept from this perspective is that acquisition of both L1 and L2 involves a process of grammaticalization in which a grammatical function (such as the expression of past time) is first conveyed by shared extralinguistic knowledge and inferencing based on the context of discourse, then by a lexical word (such as yesterday), and only later by a grammatical maker (such as the suffix -ed )