To learn a language, children must go beyond simply imitating speech and learn the rules of the language from their surrounding linguistic environment. One way to tell that children learn rules is that they apply rules to an overly broad set of words. For example, English-learning children produce forms like "breaked" or "catched" at around 3 years of age. These forms, called past tense overregularizations, show that children have implicitly discovered the past tense formation rule: namely, "add -ed to a verb to create the past tense." But does it take children until age 3 to discover such rules? A better understanding of the timeline of rule discovery will help us to develop more realistic milestones to measure children's progress in first language learning. It will also help to us understand why the past tense form is so difficult for second language learners of English and allow more informed pedagogical intervention for these learners. This work has two goals: the first is to seek evidence that the past tense rule can be found in 16-month-olds, which would be consistent with many observations that children display significant receptive abilities long before their productive abilities reach the same level. The second goal is to determine if the production of past tense overregularizations can be explained by factors other than the discovery of a grammatical rule. There is growing evidence that overregularizations in production reflect children's need to simplify their utterances as they attempt to say more and more complex sentences. Factors that may influence a child's need to simplify utterances are frequency of the lexical item and the child's age. The proposed project will combine a behavioral study with 16-month-olds and a corpus analysis of longitudinal, spontaneous child speech to achieve these goals.