In short

The trapassato prossimo is the past behind the past: it says that something was already over before another past event happened. Build it with the imperfetto of avere or essere plus the past participle - avevo mangiato, ero andato - and it means exactly what English I had eaten, I had gone means. The auxiliary and the agreement work exactly as in the passato prossimo; only the auxiliary’s tense changes.

The rule

You already know the passato prossimo (step 14): present of the auxiliary + participle. The trapassato prossimo is the same machine with one dial turned - put the auxiliary in the imperfetto (step 19) instead of the present.

ho mangiato (I have eaten) -> avevo mangiato (I had eaten) sono andato (I went) -> ero andato (I had gone)

That is the whole formation. The interesting part is when to reach for it.

• Step 1 - Check that there are two past moments. The trapassato never works alone: it needs a second, later past point to be earlier than. That point is usually a passato prossimo or an imperfetto in the same sentence, or something already established in the conversation. One past moment on its own is a plain passato prossimo.
• Step 2 - Put the earlier one in the trapassato. In Quando sono arrivato, Marco era già uscito, two things happened: Marco left, then I arrived. The arrival is the reference point (sono arrivato); the departure came first, so it takes the trapassato (era già uscito).
• Step 3 - Choose the auxiliary exactly as in step 14. Nothing new to learn. Transitive verb -> avere (avevo letto); the movement and change-of-state group -> essere (ero partito); reflexive verbs -> always essere (mi ero alzato).
• Step 4 - Apply the same agreement rule. With avere the participle is frozen in -o: Anna aveva mangiato. With essere it agrees with the subject: Anna era partita, le ragazze erano partite. If a combined or direct object pronoun comes first, it still governs the participle just as in step 30: L’avevo già vista.

The words that call for it

Certain expressions almost force a trapassato, because they exist to point backwards from a past moment: già (already), non ancora (not yet), mai (never), appena (just), prima (before), dopo che (after), perché (because, when the reason came first).

Non avevo mai visto Roma prima di quel viaggio. - the not-seeing fills all the time before the trip.

Note where the adverb sits: già, mai, ancora and appena go between the auxiliary and the participle - era già uscito, not era uscito già.

Do not over-use it

English speakers often reach for had out of habit. Italian only wants the trapassato when the anteriority actually matters to the sentence. If the events are simply told in the order they happened, a chain of passati prossimi is right: Sono entrato, ho salutato e mi sono seduto - no trapassato anywhere. Use it when you step out of order and mention something that happened earlier than where the story currently stands.

Forms & patterns

The two auxiliaries in the imperfetto, plus a participle:

One dial - the participle never changes, only the tense of the auxiliary:

Agreement, exactly as in step 14 - avere freezes, essere agrees:

Which past does what:

The two-verb frames worth memorising:

Examples

Common mistakes

• Quando sono arrivato, Marco è già uscito -> Quando sono arrivato, Marco era già uscito - two past events, one earlier than the other. The earlier one needs the trapassato; a passato prossimo would put them side by side, as if he walked out as I walked in.
• Avevo andato via prima delle otto -> Ero andato via prima delle otto - the auxiliary choice does not change in the trapassato. Andare takes essere in the passato prossimo, so it takes essere here too - just in the imperfetto.
• Anna era già partito -> Anna era già partita - with essere the participle is still an adjective in disguise and still matches the subject: -o / -a / -i / -e.
• Quando ha telefonato, mi avevo già alzato -> Quando ha telefonato, mi ero già alzato - reflexive verbs take essere in every compound tense, without exception.
• Ieri avevo visto un bel film -> Ieri ho visto un bel film - there is no second past moment here, so there is nothing to be earlier than. A lone past event is a passato prossimo, however far back it was.

Tips

• One dial, not a new tense. Say the passato prossimo out loud, then put the auxiliary in the imperfetto: ho fatto -> avevo fatto, sono uscito -> ero uscito. Everything you learned in step 14 - the essere list, the irregular participles, the agreement - carries over untouched.
• English is a reliable guide here. Wherever English says had done, Italian says the trapassato. This is one of the rare places where translating word for word works.
• It needs a partner. The trapassato is never the only past tense in view. If you cannot point to the later moment it is earlier than, you probably want the passato prossimo instead.
• già, mai, ancora, appena go in the middle: era già uscito, non avevo mai visto, non era ancora tornato, mi ero appena alzato. Squeezing them between the two words is the natural Italian rhythm.