Three case-studies, using longitudinal records of children's spontaneous speech, illustrate what happens when a child's syntax changes. The first, examining acquisition of English verb-particle constructions, shows a near-total absence of commission errors. The second, examining acquisition of prepositional questions in English or Spanish, shows that children (i) may go as long as 9 months producing both direct-object questions and declaratives with prepositional phrases, before first attempting a prepositional question; and (ii) at some point, abruptly begin producing prepositional questions that are correctly formed for the target language. The third case study shows that in children acquiring English, the onset of verb-particle constructions occurs almost exactly when that child begins using novel noun-noun compounds. After a discussion of the implications for the nature of syntactic knowledge, and for the mechanisms by which it is acquired, two examples are presented of as-yet untested acquisitional predictions of parametric proposals in the syntax literature.