le futur antérieur

Formation

The future perfect (futur antérieur) is formed with the simple future of the auxiliary (either être or avoir), plus the past participle of the main verb. The choice between être and avoir as an auxiliary is the same as in the passé composé.

Future perfect = auxiliary in the future + past participle of main verb

Note that with the auxiliary être, the past participle agrees in number and gender with the subject. 

Usage

The futur antérieur is used for an action that precedes, or will be completed before, another action in the future. While the futur is expressed in English by 'will' + main verb (will finish), the futur antérieur is usually translated as 'will have' + past participle of verb (will have finished). For example:

Negation is formed as usual by placing ne / n' ... pas around the conjugated verb, which in this case is the auxiliary: 

Nous aurons fini le pop-corn avant la fin du film. We will have finished the popcorn before the end of the film. 

Tu es en retard! Le train sera parti avant ton arrivée à la gare! You’re late! The train will have left before your arrival at the train station.

conjunctions

The futur antérieur is used similarly in French and English. However, French uses the future perfect after certain conjunctions like quand and lorsque (when) or dès que and aussitôt que (as soon as) where English would use the present tense.

In the examples above, the subordinate clauses starting with quand and dès que are in the futur antérieur t because the action of the subordinate clause will be performed before the action in the main clause.