Futur II for assumptions about the past (German future perfect as inference)

At an advanced level you will meet Futur II far more often as a tool for speculation about the past than as a literal future tense. The form is always wird + Partizip II + haben/sein: Er wird es vergessen haben does not mean "he will forget it in the future" but "he must have forgotten it" — a confident inference about a completed event.

This reading is very common with the particles wohl and sicher(lich), which underline the speaker's guess: Sie wird wohl krank gewesen sein ("she will probably have been ill"). The meaning is subjective certainty, close to müssen in its inferential use, which is why this topic pairs naturally with the subjective modal verbs.

Keep two things apart. First, the literal future-perfect use still exists: Bis morgen wird er die Arbeit beendet haben ("by tomorrow he will have finished the work") describes a future point by which an action is complete. Second, the choice between haben and sein as the closing auxiliary follows the ordinary perfect rule — transitive and most other verbs take haben (vergessen haben, verpasst haben), while verbs of motion and change of state take sein (gewesen sein, gegangen sein). A time adverbial usually disambiguates the two readings.