imperfetto of avere o essere + past participle
Trapassato expresses an action that took place before antoher past action or point in time.
The more simple past can be expressed in the passato prossimo or the imperfetto.
In English, you would use “had” plus a past participle to make the Italian equivalent of Trapassato.
My grandparents had already left when I called.
I nonni erano già usciti quando ho telefonato.
I was tired because I had been swimming all morning.
Ero stanca perché avevo nuotato tutta la mattina.