The aim of this study is to generate knowledge about what 6th grade students (12-13 years old) need to discern in order to be able to use the English progressive aspect (PROG) in a syntactically and semantically accurate way. This object of learning is not only complex, but it is unmarked grammatically in Swedish, which poses considerable difficulties for English language learners. The theoretical framework has been the Variation Theory of learning which was used to design and analyse teaching and learning. A basic assumption is that learning is being able to discern critical aspects by seeing and experiencing variation and not sameness. The method chosen to answer the research question was Learning Study, an interventionist, iterative classroom-based approach, characterized by the double aim of improving teaching and at the same time developing theory. Three secondary school English teachers and the teacher researcher, collaborated to plan, teach, evaluate and analyze a series of six lessons with the Variation Theory as the pedagogical principle. Empirical data consisted of interviews, pre- and post-lesson assessments, and video recordings of the lessons. The results show that the students in this study needed to discern the following four critical aspects: (1) to differentiate between tense and grammatical aspect, (2) to differentiate between simple aspect and progressive aspect, (3) to discern the concept of ongoingness and (4) to differentiate between stative and non-stative meanings. Furthermore, the findings also suggest that (1) the necessity of separation, (2) treating the PROG as an undivided whole and (3) the use of carefully cho-sen and powerful examples encourage an enhanced understanding of the PROG.