The trapassato prossimo (past perfect) is a compound tense used to express an action that took place before another past action.
How it is formed
It is formed by using the imperfetto of the auxiliary (avere or essere) and the past participle of the main verb.
Prima di andare a Ferrara, avevochiamato questa ditta di Modena.
Before going to Ferrara, I had called this company in Modena.
Agree?
For the trapassato prossimo, as for all compound tenses, the use of the auxiliary is determined by the main verb. When the trapassato prossimo is formed with essere, the participio passato (past participle) must agree in gender and number with the subject of the verb.
Trapassato prossimo for lavorare and andare
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Helping adverbs
It is very common to use adverbs like già (already), non… ancora (not yet), non… mai (never), etc., with the trapassato prossimo.
