A future that has nothing to do with the future
Someone asks you where Marco is. You do not actually know, but you would bet on home. In Italian you answer Sarà a casa — literally he will be at home, but what it means is he must be at home. Right now, at this moment.
This is the futuro epistemico, the future of conjecture. The future form here signals one thing only: I am not certain, I am inferring. English has a trace of it too: That'll be the postman. But in English it is a small corner of the language, in Italian it is the everyday way to guess.
Two everyday guesses
Dov'è Marco? Sarà a casa.
Where is Marco? He must be at home.
Non è venuto. Avrà dimenticato l'appuntamento.
He didn't come. He must have forgotten the appointment.
Guessing about now — futuro semplice
You are talking about a state that holds at this moment, but you are not sure. The present tense asserts, the future infers: you have clues but no certainty.
Same content, two degrees of certainty
Anna ha trent'anni.
Anna is thirty. (I know)
Anna avrà trent'anni.
Anna must be about thirty. (I reckon)
Luca è in ufficio.
Luca is at the office. (I know)
Luca sarà in ufficio.
Luca must be at the office. (I reckon)
Situations where people talk like this
Quanto costa? Non lo so, costerà sui venti euro.
How much is it? No idea, it must be around twenty euros.
Non è ancora arrivata. Sarà bloccata nel traffico.
She isn't here yet. She must be stuck in traffic.
Il cane abbaia. Ci sarà qualcuno alla porta.
The dog is barking. There must be someone at the door.
Guessing about earlier — futuro anteriore
Move the guess into the past and the form moves with it: instead of the simple future you take the futuro anteriore. It stands where the passato prossimo would otherwise stand. English does the same: must be becomes must have been. Sarà stanco is he must be tired, Sarà stato stanco is he must have been tired.
From fact to guess
Working backwards
Non è qui. Avrà perso il treno.
He isn't here. He must have missed the train.
Ha gli occhi rossi. Avrà pianto.
Her eyes are red. She must have been crying.
Non ha detto niente. Non avrà capito la domanda.
He said nothing. He can't have understood the question.
In a negative guess English swaps must for can't: he can't have understood. Italian just negates the same future, so must have and can't have both come out as one form.
After quando, appena or dopo che, the same form means a real future instead: Quando avrà finito, uscirà — When he has finished, he'll go out. That is not a guess.
Questions and concessions
The guessing future also appears in questions. You are not really asking anybody, you are puzzling out loud. English does this with I wonder… or who on earth…, a whole extra clause where Italian just bends the verb.
Wondering aloud
Chi sarà a quest'ora?
Who on earth can that be at this hour?
Dove avrò messo gli occhiali?
Where on earth did I put my glasses?
Si sarà arrabbiato per ieri?
I wonder if he got annoyed about yesterday.
With anche or pure straight after the verb, the future grants a concession, usually followed by ma or però: Sarà anche bravo, ma non mi piace — he may well be good at his job, but I don't like him.
Costerà anche poco, ma è brutto.
It may well be cheap, but it's ugly.
Avrà pure ragione, però esagera.
He may well be right, but he's going too far.
And in English?
English marks a guess with a separate word, usually a modal: He must be at home, he might be at home, he's probably at home. Take it away and the sentence is a plain assertion. In Italian the guess lives in the verb, and without the future È a casa is a statement of fact.
How to render it in English
must is the default: Sarà uscito — he must have gone out.
probably, I expect when must would sound too strong: Avrà avuto un imprevisto — something has probably come up.
about, around with times, prices and ages: Saranno le tre — it must be about three.
