English has three main groups of tenses, past, present and future. The past tense in general deals in actions that have already occurred. But it can also be split into various subdivisions which use the verb phrase to convey subtle difference in when events happened and how the action is regarded. These subdivisions can be roughly defined as past perfect and past continuous tense or the imperfect tense.
Imperfect and past tense verbs show the difference in the timing of past actions The past perfect tense relates to an action that was completed at a time relative to another timed event. For example, "By 2000 they had married." The imperfect tense, refers to an action that continued for some time. And example might be "I was living there before 2005."
The aspect of a verb phrase shows how the speaker of a sentence regards the action communicated by the main action verb, whether it was past perfect or imperfect. In English, this involves the use of the verbs "to have" or "to be" followed by the main action verb of the sentence to convey a subtle difference in the timing of an action. The imperfect tense is identified by the use of the words "was" or "were" which show that the past action was continuous; for example, "I was punching." The past perfect uses "has," "had" or "been" to show that the action was completed at a set point in time; for example, "I had punched."
By their different applications of timing and aspect, past perfect and imperfect verb phrases are used to show the varying ways that actions occurred in the past. The imperfect tense is used to describe a completed event that occurred over a period of time or an event that occurred within an event. The perfect tense is used to show a brief action completed at a single point in time or to show which of two events occurred before the other.
Examples help to illustrate the subtle difference between the perfect past tense and the imperfect tense in action:
Simple past tense -- I swam/we swam
imperfect tense -- I was swimming/we were swimming
perfect past tense --I had swum/ we had swum
past perfect continuous -- I had been swimming/they had been swimming