This pilot study investigated university-level intermediate Spanish learners’ (n=16) explicit knowledge about a contrast in grammatical aspect (preterite/imperfect) after they received rule-based explicit instruction. Prior studies have found that learners’ explicit knowledge about preterite and imperfect includes partially understood rules, which can have long-lasting effects on their performance, so it is important to investigate the disconnect between what is taught and what is learned. Learners completed a cloze test and then introspected about their performance. The data were analysed qualitatively and compared to verbalisations of L1 Spanish speakers (n=6). Four main themes emerged: learners failed to learn the concept of grammatical aspect, they demonstrated confusion about lexical aspect, they attributed the difference between preterite and imperfect to a binary system of lexical aspect feature complexes based on duration of the verb/predicate, and they relied on self-developed categorical rules about adverbial markers. Suggestions for modifications to the conventional pedagogical rules are offered.