The study aimed, firstly, to assess a group of Indonesian adult EFL students' mastery of tenses and aspects as part of their mastery of English grammar and, secondly, to identify if their experience of going through the instructional processes, their perceptions of and habits in studying English grammar shaped their mastery of tenses and aspects. The data were collected through a grammar test and a semi-structured interview. As the study was conducted, the students were sitting in the fourth (last) year of their study. Despite having such long instructional experience, however, the findings show that the mastery of tenses and aspects of majority of the students still fell into the "fail" category. Also, their having perception that studying English grammar is important apparently did not shape their mastery of tenses and aspects. What seemed influential as to shaping their mastery was much more related to their habits in studying, the modes of instruction they experienced, and the type of language samples they were exposed to, especially during the process of receiving grammar instruction in the classroom.