What is a past imperfect tense?

The past imperfect tense (or just imperfect tense) is used to describe an action in the past that is recognized as unfinished or continuous, which contrasts that of the preterite tense which recognizes an action in the past as being completed. English doesn't have an imperfect tense. A rough example of the imperfect tense in English would be "I was reading". The verb "was" implies that although I was reading sometime in the past, I didn't necessarily finish or the action got interrupted.