It is frequently argued that children are competent with some dimension of language, but their knowledge is being masked by performance limitations. However, in most cases, the evidence for these performance factors is indirect and the specific links between cognitive skills and linguistic forms is vague. The current work examines a well-documented under-extension in children’s language and the cognitive skills that predict children’s performance of it. The linguistic phenomenon involves aspect: children prefer to say (and better comprehend) predicates describing bounded events with perfective rather than imperfective morphology and the reverse for unbounded events. That is, despite the fact that all four of the following sentences are grammatical, children prefer “The girl closed the door” over “The girl was closing the door” and “The girl was listening to music” over “The girl listened to music”. Children and adults were tested on their ability to understand a range of aspectual combinations (both preferred and non-preferred) and were also tested on a series of independent cognitive assessments. The results showed specific links between inhibitory control and vocabulary size with different non-preferred combinations that were consistent with formal semantic accounts of those linguistic forms. More generally, the results show how it is possible to use performance to illuminate the nature of competence.