Three studies using the intermodal preferential looking paradigm examined onset of productive comprehension of tense/aspect morphology in English. When can toddlers understand these forms with novel verbs and novel events? The first study used familiar verbs and showed that 26-36-month olds correctly matched a past/perfective form ("-ed" or irregular past) to a completed version of an event and a present/imperfective ("is V-ing") to the ongoing version of the same event. The second study used novel verbs and events and found that 33-month olds failed to use tense/aspect morphology to choose between completed and ongoing versions of the same event. The third study also used novel verbs and events but simplified the processing demands of the task in several ways (using initial priming of the events and classes of meaning, using different events within test pairs). This study found that 30-month olds successfully used tense/aspect morphology to choose between ongoing and completed novel events. The results demonstrate that children have productive command of tense/aspect morphology by 30 months and have therefore begun the process of creating an abstract grammar containing this element. (Contains 5 figures and 5 tables.)