Complex sentence: The imperfective (abbreviated or more ambiguously) is a grammatical aspect used to describe a situation viewed with internal structure, such as ongoing, habitual, repeated, and similar semantic roles, whether that situation occurs in the past, present, or future.
Split into simpler sentences: The imperfective (abbreviated or more ambiguously) is a grammatical aspect. It refers to a situation viewed from a particular viewpoint as ongoing, habitual, repeated, or generally containing internal structure, whether that situation occurs in the past, present, or future.