This article presents a generative analysis of the acquisition of formulaic language as an alternative to current usage-based proposals. One influential view of the role of formulaic expressions in second language (L2) development is that they are a bootstrapping mechanism into the L2 grammar; an initial repertoire of constructions allows for statistical induction of abstract grammatical categories, such that formulaic language is the data source from which syntactic rules are derived. This study brings evidence to bear on this debate from three studies of the acquisition of conventional expressions by L2 learners of English. A total of 271 learners and 58 native speakers completed either an oral conversation-simulation task or an aural-written elicited imitation task. The data show that while learners exhibit knowledge of both contextualized use and the lexical core of conventional expressions, production data reflect the morphosyntactic knowledge of learners at particular stages of development. Formulaic language does not drive the acquisition of syntax; rather, the acquisition of syntax as an independent process drives changes in the production of conventional expressions. Their gradual transformations allow for insights not only into the acquisition of syntax, but also into the nature of multi-word expressions in the mental lexicon.