Investigating the L2 temporality, most previous studies within the Aspect Hypothesis framework focused on the basic meanings or prototypical uses of past morphology. The present study, however, including other less prototypical uses of past morphology, addresses 2 questions: (i) how the uses of simple past and past progressive morphology change as learners become more proficient in their target language; (ii) to what extent lexical aspectual class and L1 influence the uses of simple past and past progressive morphology. Using a cloze test as an elicitation task, this study analyzes data from 5 groups of Thai EFL learners at different proficiency levels. Results show that learners use past morphology more accurately as their L2 proficiency levels increase. The tense-aspect marking was, however, affected by lexical aspectual class. Learners first use simple past form on telic verbs, eventually extending its use to atelic verbs. The progressive form, on other hand, begins with atelic verbs and then extends to telic verbs. All learner groups, however, exhibit a higher rate of appropriate use of past morphology in the more prototypical uses than in the less prototypical uses. Additionally, L1 plays an important role in the tense-aspect marking. Learners at different proficiency levels, however, use different L1-influenced forms, suggesting that L1 influence is constrained by L2 development. Contributing to the body of research on L2 tense-aspect, this study sheds light on the nature of difficulty learners experience in developing L2 tense-aspect system.