This article reports an empirical study that examines to what extent learners can identify and understand the grammatical structures they produce when they speak spontaneously. In the study, 20 upper-intermediate Polish learners of English were interviewed in English by the researchers. The structures used accurately by each learner were isolated and each of the participants was then administered a separate test. The task in the test was first to identify correct sentences and then to provide relevant grammar rules. The results show that in most cases, the learners were able to identify and explain the grammar rules that accounted for their own accurate L2 performance. In terms of second language acquisition (SLA) theory, this means that there were few grammatical structures or categories that the learners knew only implicitly. For teachers, the study indicates that explicit grammar rules can, in an indirect way, contribute to SLA.