The Trapassato Prossimo (Past Perfect) is used to describe an action that had already happened before another action in the past took place. Think of it as the "past of the past".

How to form it

It is formed exactly like the Passato Prossimo, but you use the Imperfetto of the auxiliary verb (avere or essere) instead of the Present tense.

• Avere: avevo, avevi, aveva, avevamo, avevate, avevano + Past Participle (e.g., avevo mangiato)
• Essere: ero, eri, era, eravamo, eravate, erano + Past Participle (e.g., ero andato/a)

Note: The rules for choosing avere vs. essere are the same as for the Passato Prossimo. And if you use essere, the past participle must agree in gender and number with the subject!