Future perfect

Relatively rare in English, the future perfect serves to express one future action which precedes a future moment or another future action. Moreover, it asserts that these actions will be completed before the principal action. It is formed by adding the modal "will" al auxiliary "have," preceding the past participle:

• She will have finished before eight o'clock.
• Tomorrow morning they will all have left.
• They will already have finished eating by the time we get there.

One can often use the simple future instead of the future perfect, but a nuance is lost: the simple future does not emphasize the completion of the first action:

• Tomorrow morning they will all leave. (The future perfect would emphasize that they will already have departed before tomorrow morning.)
• They will finish eating by the time we get there. (They may finish just as we arrive; the future perfect would emphasize that they will have finished before we arrive.)