Future Perfect

Conjugate werden, then add the past participle, then the infinitive haben or sein at the end. Verbs that take haben in the Perfekt keep haben: ich werde gemacht haben. Motion and change-of-state verbs take sein: ich werde gegangen sein. The auxiliary infinitive always closes the sentence.

For two things. First, completion before a future deadline: Um 18 Uhr werde ich gegessen haben, by six I will have eaten. Second, and more often in speech, an assumption about something already finished: Sie wird es vergessen haben, she has probably forgotten. Context and particles like wohl separate the two.

It means he has probably already left. This is Futur II used for a supposition about the past, not about the future. The particle wohl reinforces the guess. German speakers use this construction far more for probability than for genuine future completion, which is why it sounds so idiomatic.

Use whichever auxiliary the verb takes in the Perfekt. lesen takes haben, so werde gelesen haben. gehen takes sein, so werde gegangen sein. The same applies to fahren, kommen, bleiben, laufen and fliegen, all verbs of motion or change of state that select sein.

The assumption use is common, the deadline use much less so. For real future completion Germans often prefer the present tense with a time phrase: Bis morgen bin ich fertig instead of Bis morgen werde ich fertig sein. But Das wird wohl stimmen, meaning that is probably true, is heard constantly.