In English the past perfect tense (trapassato prossimo) is formed with the auxiliary "had" the past participle of the main verb. In Italian, the trapassato prossimo, a compound tense, is formed with the imperfetto of the auxiliary verb avere or essere and the past participle of the acting verb.

The students were tired because they had studied until late. He didn't go to the theater because he had already seen the film. The past perfect tense (trapassato prossimo) is used when two actions happened at different times in the past.

Here are a few examples of the trapassato prossimo:

Già erano partiti quando sono arrivato. (They had already left when I arrived.)

Avevo chiuso le finestre quando è cominciato a piovere. (I had shut the windows when it started to rain.)

La macchina sbandava perché aveva piovuto. (The car was sliding because it had rained.)