A future tense that talks about the past
The Futur II is built like the English future perfect (he will have forgotten), but in everyday German it mostly means something else: something is already over, and here is my confident guess about it. A real future perfect needs an explicit deadline.
Sie wird lange gearbeitet haben.
She will have worked late.
Sie wird schon losgefahren sein.
She will have set off already.
Ich hoffe, dass sie den Vertrag unterschrieben haben wird.
I hope she will have signed the contract.
In a subordinate clause the conjugated wird goes last.
Use 1: a guess about something finished
This is by far the commoner use. You see a result and reason back to the cause. The best English match is must have, not will have. Words like wohl, sicher, bestimmt and schon make the guess explicit.
Der Zug wird Verspätung gehabt haben.
The train must have been late.
Es wird nachts geregnet haben, die Straße ist nass.
It must have rained in the night; the road is wet.
Das Paket wird schon angekommen sein.
The parcel will have arrived by now.
Use 2: finished before a point in the future
This reading needs a time phrase that sets the deadline: bis Freitag, in zwei Wochen, bis du zurück bist. Without one, the sentence is read as a guess. It sounds formal; in speech the Perfekt with the same time phrase is more usual: Bis Montag habe ich den Bericht geschrieben.
Bis Freitag werde ich alles erledigt haben.
By Friday I will have got everything done.
Bis Ende des Monats werden wir umgezogen sein.
By the end of the month we will have moved.
