The future perfect: j'aurai fini, elle sera partie

The futur antérieur (future perfect) says what will already be over by a point in the future: J'aurai fini avant midi. - I will have finished before noon. It is the futur simple of avoir or être plus a past participle, and it has three jobs.

If you can build the futur simple and the passé composé, you can build this tense today. Job 1 is the future perfect English also has; for job 2 English uses a present perfect (when you have finished); job 3 has no English tense at all.

How to form it

Take the passé composé and put its auxiliary into the future.

The glosses: I will have finished, I will have left, and so on down. The auxiliary takes the future endings you already know: -ai, -as, -a, -ons, -ez, -ont.

The auxiliary choice and the agreement rules are the passé composé's, unchanged:

Negation wraps the auxiliary, as in every compound tense, and an object pronoun sits in front of it: Il ne l'aura pas vu. - He will not have seen it.

Done by then

The first job is the one English shares: an action that will be complete before a point still in the future. A time phrase usually names the point: avant demain, dans deux heures, d'ici la fin du mois, à midi.

• Avant demain, j'aurai terminé le dossier. - By tomorrow I will have finished the file.
• Dans deux heures, ils seront arrivés. - In two hours they will have arrived.
• D'ici la fin du mois, nous aurons déménagé. - By the end of the month we will have moved.
• À midi, elle sera partie. - By noon she will have left.

Set it beside the futur simple and the difference is when the action happens:

The first puts the arrival at the two-hour mark; the second puts it before the mark and looks back at it.

After quand and dès que

The tense is most at home after a time conjunction, where one future action must be over before the next can start.

• Quand tu auras fini, on partira. - When you have finished, we will leave.
• Dès qu'il sera arrivé, je t'appellerai. - As soon as he has arrived, I will call you.
• Une fois que nous aurons vendu la maison, nous chercherons un appartement. - Once we have sold the house, we will look for a flat.

English puts a present perfect there (when you have finished). French will not: the finishing is in the future, so its verb is too. The whole rule, and when the plain futur simple is enough, is on quand plus the future.