The article discusses the potential of combining learner corpus research with experimental studies in order to fine-tune the understanding of learner language development. It illustrates the complementarity of the two methodological approaches with data from an ongoing study of the acquisition of the English tense and aspect system by French learners. The first research question relates to the development of accuracy of tense and aspect usage in written production over time; longitudinal learner corpus data is used to answer this question. Its results are then incorporated into an experimental study that addresses the second research question, which seeks to uncover reasons why some tense and aspect features remain difficult to master even for advanced L2 learners. The first part of the article reviews the status of longitudinal research in Second Language Acquisition (SLA) and describes the Longitudinal Database of Learner English (Longdale) used for the study. The second half of the article presents the results of the learner corpus analysis. These reveal that over a period of 3 years, learners' tense and aspect errors decrease. However, the English progressive continues to present considerable learning difficulties. Two follow-up experiments investigated which elaborations of the progressive epistemic schema L2 learners continue to find difficult. They showed that advanced learners master the most salient elaboration of the present progressive (i.e., ongoingness, including extended ongoingness). By contrast, their understanding of less core uses (i.e., planned events) is much less precise. The article concludes with suggestions for the teaching of tense and aspect to advanced EFL learners. (Contains 11 tables, 2 figures, and 10 notes)