Ich muss eingeschlafen sein. Why is the sentence built this way?

Question

It's a sentence I've encountered in a game. I'm quite sure the meaning is: I must have fallen asleep. (The character is found sleeping, wakes up and says: Hahaha, tut mir Leid! Ich war so aufgeregt, dass ich schon seit dem Morgengrauen hier auf dich warte. Ich muss wohl eingeschlafen sein.) So I understand the meaning but I can't figure out why the verbs are conjugated the way they are.

My question is why is muss in the present tense? And why is sein in the infinitive mood? Or did I get the meaning wrong?

Answer 1

It's built that way because it's the correct way to have a present modal verb (muss) with a present perfect infinitive (eingeschlafen sein).

Ich bin eingeschlafen translates to I have fallen asleep (present perfect tense). The auxiliary verb to form the perfect of einschlafen is sein, not haben. The infinitive of einschlafen in present perfect thus is eingeschlafen sein.

Müssen is the auxiliary verb to express epistemic modality ('from the circumstances it seems the only logical conclusion that I have fallen asleep'). The meaning is the same as of English must in must have fallen asleep. As every modal verb, it requires an infinitive as supplement by definition. This is where the present perfect infinitive from above comes into play.

The combination of auxiliary + present perfect infinitive is the sentence you already know:

Ich muss _{(modal auxiliary, finite)} {eingeschlafen _{(past participle)} sein _{(present perfect auxiliary, infinite)}}_{present perfect infinitive}.

Note that in English, due to the relatedness of both Germanic languages, it's exactly the same -- the modal auxiliary is finite, the perfect auxiliary is infinite as this third person example shows:

He must (finite) have (infinite, otherwise it would be has) fallen (perfect participle) asleep.

Must₁ have₂ fallen_3a asleep becomes muss₁ eingeschlafen_3b sein₂ (because German is a object-verb, not verb-object language).