This study expands on previous research in concept-based instruction (CBI) by investigating the use of the grammatical concept of aspect as a tool to mediate decision making during tasks. Specifically, this article reports the findings of a longitudinal case study that examines one student's explanations for his choice of French past tense ("passé composé" or "imparfait"). The audio-recorded data comprise researcher/student discussions on the student's written responses to tasks, which the student performed before and after a one-hour lesson on the concept of aspect. Findings indicate an evolution in the student's explanations. Before the lesson, the student uses formal rules of thumb, but conceptual terminology appears in his explanations immediately after the lesson. Four weeks after the lesson, he justifies his choices by focusing primarily on the meanings he wishes to communicate. Most important, he relies on his own developed method, based on logical thinking, to explain his decisions on verb forms. The article discusses the pedagogical implications of the findings, suggesting that the teaching of written courses at the tertiary level might be the most appropriate context for introducing CBI.