The French future perfect is most commonly used like the English future perfect: to describe an action that will have happened or will be finished by a specific point in the future.
J'aurai mangé à midi.
I will have eaten at noon.
Quand tu arriveras, il l'aura déjà fait.
When you arrive, he will already have done it.
Elle lui aura parlé demain.
She will have talked to him (by) tomorrow.
Dans un mois, nous serons partis.
In a month, we will have left.
There are three uses of the French future perfect that don't correspond to the English future perfect:
1. In subordinate clauses that begin with the conjunctions aussitôt que, dès que, lorsque, quand, une fois que, and après que, the future perfect is used to express a future action which will be completed before the action in the main clause. In English, a present tense or past tense would be used here.
Quand je serai descendu, tu pourras me le montrer.
When I have come down, you can show it to me.
Nous le ferons aussitôt qu'elle sera arrivée.
We'll do it as soon as she arrives / has arrived.
2. The future perfect can make simple assumptions regarding past events, where the English modal verb "must" would be used in conjunction with the past perfect:
Pierre n'est pas ici ; il aura oublié.
Pierre isn't here; he must have forgotten.
Luc est heureux ; il aura gagné.
Luc is happy; he must have won.
3. In historical narratives, the events of a person's life can be described with the future perfect even though those events have long since passed. In English, these might be translated by a past tense or conditional:
Napoléon aura pris une décision importante.
Napoleon made / would make an important decision.
George Sand aura écrit le roman La Mare au Diable en quatre jours.
George Sand wrote / would go on to write the novel La Mare au Diable in four days.
Go on to page 2 to learn how to conjugate the future perfect.
Quiz: French future perfect