In short

The futuro anteriore is the future of essere/avere plus a past participle (avrò finito, sarò partito), and it does two jobs. It marks the earlier of two future actions - Quando avrò finito, ti chiamo - and, far more often in speech, it guesses about the past: Sarà partito ieri (he must have left yesterday). The simple future does the same guessing about the present: Saranno le otto (it must be about eight).

The rule

You already know the simple future from step 21, where it was presented as pure time: later. That was only half the story. Both futures also work as guessing tenses, and that is where this lesson earns its keep.

Step 1 - Build the form. Take the futuro semplice of the auxiliary and add the past participle: avrò parlato, sarò andato. Nothing here is new. The auxiliary is the same one the verb takes in the passato prossimo - avere for most verbs, essere for the movement/change-of-state group - and with essere the participle still agrees with the subject: Anna sarà partita, i ragazzi saranno partiti.

Step 2 - Job one: anteriority. When a sentence holds two future actions, the one that finishes first takes the futuro anteriore; the other takes the simple future or the present. The trigger conjunctions are quando, dopo che, appena, non appena, una volta che:

Quando avrò finito il lavoro, ti chiamo. The finishing comes first, the call second. English is content with when I have finished, but Italian keeps the whole sentence pointed at the future.

It also works with no clause at all, measured against a future point in time: Fra due anni avrò finito l’università.

Step 3 - Job two: supposition. This is the frequent one. Italian has no everyday equivalent of English must be / must have: instead it puts the sentence in the future and lets the tense carry the doubt. The rule is a simple shift of one step:

• To guess about now, use the futuro semplice: Marco sarà stanco. (Marco must be tired.)
• To guess about something already over, use the futuro anteriore: Marco sarà partito. (Marco must have left.)

So the pair presente / passato prossimo has a mirror image in futuro semplice / futuro anteriore - same relationship, moved into guess-mode. Nobody is talking about the future in Saranno le otto: the speaker is looking at a clock right now and estimating.

The same forms turn a question into wondering aloud rather than a request for information: Che ore saranno? is I wonder what time it is, not a question you expect answered.

A minor extension of the same value: put the future in front of a concession and it means granted, maybe: Avrà anche ragione, ma non mi convince. (He may well be right, but he doesn’t convince me.)

Guessing with the conditional - Sarebbe partito - is a different tool with a different meaning, and it belongs to step 45.

Forms & patterns

The two auxiliaries in the future, plus a participle:

Fact on the left, guess on the right - the tense is the only thing that changes:

Which tense goes where when two future actions meet:

The conjunctions that introduce the earlier action: quando, dopo che, appena, non appena, una volta che.

Examples

Common mistakes

• Dopo che ho mangiato, uscirò. -> Dopo che avrò mangiato, uscirò. English is happy with after I have eaten, but Italian keeps both halves in the future and marks the earlier action with the futuro anteriore.
• Quando avrò finito, ti avrò chiamato. -> Quando avrò finito, ti chiamo. Only the earlier of the two actions takes the futuro anteriore. The main clause takes the present or the simple future - never a second futuro anteriore.
• Quando avrò arrivato a casa, ti scrivo. -> Quando sarò arrivato a casa, ti scrivo. The auxiliary is whichever one the verb already takes in the passato prossimo. Arrivare takes essere, here and everywhere.
• Le ragazze saranno arrivato tardi. -> Le ragazze saranno arrivate tardi. With essere the participle agrees with the subject, exactly as in the passato prossimo. avere freezes it, essere agrees.
• Dov’è Marco? Partirà. -> Dov’è Marco? Sarà partito. To guess about something already over you need the futuro anteriore. Partirà guesses about now or promises a departure - it cannot mean he must have left.

Tips

• The guess is a shift of one step. Whatever tense states the fact, move it one notch into the future to state the hunch: è stanco -> sarà stanco, è partito -> sarà partito. English does the identical trick with must: he is tired -> he must be tired.
• Anteriore means “the one that comes first”. Despite the name, it is the earlier action, not the later one. If you can insert first into the English (when I’ve first finished…), that clause is your futuro anteriore.
• In speech, the guessing use is the one you actually hear. The temporal futuro anteriore is common in writing, but spoken Italian often sidesteps it (Quando finisco, ti chiamo). The suppositional one, on the other hand, is everywhere - including the stock reply Sarà! (If you say so!).
• Nothing new to memorise. Auxiliary choice, irregular participles and agreement are all imported unchanged from the passato prossimo; the only new part is putting avere/essere into the future you already learned.