What the Futuro Perfecto is for
The Futuro Perfecto has two uses. It says what will be already finished by a point in the future, exactly like the English future perfect: by tomorrow I will have spoken to the boss. And it expresses a guess about the past, where English reaches for must have: he must have left already.
The two uses
Finished by a point in the future
The point in time is usually given: with para (para las ocho, para entonces), with ya, or in a cuando clause.
Para las ocho habré terminado el informe.
By eight I will have finished the report.
Cuando llegues, ya habremos comido.
By the time you get here, we will already have eaten.
En junio habré vivido diez años aquí.
In June I will have lived here for ten years.
Guessing about the past
You guess at something you would otherwise state in the present perfect: Ha salido becomes Habrá salido. English says must have, and British English also has he'll have left by now.
Habrá salido ya.
He must have left already.
Habrán perdido el tren, por eso tardan tanto.
They must have missed the train, that's why they're taking so long.
¿Dónde habré puesto las llaves?
Where on earth can I have put the keys?
Guesses about the present use the plain future (estará), guesses about the past with a clear point in time (ayer, cuando llamé) usually the condicional (estaría).
Don't confuse habrá, habría and había
Three forms of haber sound alike but belong to different tenses, and English keeps them apart with three different words. Listen to the ending.
habrá llegado
he must have arrived (Futuro Perfecto)
habría llegado
he would have arrived (Condicional Compuesto)
había llegado
he had arrived (Pluscuamperfecto)
