The aspect hypothesis (Andersen & Shirai, 1994) proposes that language learners are initially influenced by the inherent semantic aspect in the acquisition of tense and aspect (TA) morphology. Perfective past emerges earlier with accomplishments and achievements and progressive with activities. Although this hypothesis has been extensively studied, there have been no analyses of the frequency, form, and function of relevant types and tokens in the input. This article reports the results of 2 corpus-based studies investigating how various features of the input--frequency distributions, reliabilities of form-function mapping, and prototypicality of lexical aspect--affect TA morphology. Study I determined the relative frequency profiles of exemplars of English TA and employed various statistics to determine the associations between particular verb-aspect combinations. Study II expanded the aspect hypothesis, examining whether native speakers judge the most frequent forms in isolation to be more prototypical in lexical aspect. Analyses were then matched against acquisition data for different TA patterns by adult learners of English (Bardovi-Harlig, 2000) to determine whether the verbs' acquisition order is determined by their frequency, form, and function in the input. Rather than testifying to the effect of 1 factor alone, the results suggest that frequency, distinctiveness, and prototypicality jointly drive acquisition.