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" to the 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.)