This study employs corpus-based and computer-aided methodology to investigate Polish advanced learners' use of the past progressive in L2 English. The analysis is performed within the framework of the Aspect Hypothesis and it is based on narrative and descriptive essays (the total of 35,319 tokens) drawn from the PELCRA learner corpus. The learner data is contrasted with two sections of the FLOB corpus comprising native speakers' texts of a comparable genre. The results have demonstrated that, although Polish advanced learners on the whole can dissociate the use of tense-aspect morphology from its prototypical combinations with the lexical aspect and their use of the past progressive across the situation types reflects the native patterns, they tend to overuse past progressive forms in comparison to the native norm. This tendency can be explained by L1 influence, but it is also reinforced by writing instruction, which fails to sensitise students to the stylistic effects of employing preterite versus past progressive forms to describe the background situation in a narrative.