Abstract: Three case studies—determiners, subjects, and the active–passive relation—are used in this chapter to argue that children’s syntactic knowledge is greater than it appears on the surface. Even when most of children’s speech consists of two-word utterances, their grammars contain genuinely syntactic categories, plus operations that combine and move those categories in ways that are isomorphic with adult grammar. Diagnostic tests can be used to determine if a child has the knowledge at issue, and tests of limited executive functions could help explain why the child’s output seems at variance with that knowledge. The child can use at least two methods to determine what not to include in his or her utterances. The child can use information structure and exclude low-information elements, such as determiners that are not essential for meaning. The child can use already established prosodic structures to fit his or her utterance to, resulting in a failure to include elements that do not fit the prosodic template, such as initial pronominal subjects. With evidence for both competence and performance factors, the child’s behavior can be explained.