The purpose of this study was to investigate how retrospective verbal reports can contribute to our understanding of the factors influencing the interlanguage development of tense and grammatical aspect. The goal was to gain insights into (1) the mental representations second language learners hold of tense and grammatical aspect, and (2) the factors influencing how knowledge of temporal morphology develops and is accessed during real-time production. The background data which inform the present investigation come from a series of studies involving over 200 adult learners of English from different L1 backgrounds. The elicitation instruments include controlled written production and timed judgement tasks. This study reports on the findings from the retrospective interview protocols which encouraged learners to reflect on their performance on the controlled tasks. Participants articulated their own hypotheses about how temporal morphology works in English and offered their perceptions of factors influencing how they process and interpret tense-aspect forms. Two key factors emerged from the verbal reports: learners' awareness of semantic categories and their sensitivity to frequency effects in instructional input. Both appeared to constrain learners' access to new knowledge that would allow their interlanguage to develop to more advanced levels.