Ich arbeite oft bis spät in die Nacht.

Why is it Akkusativ? I think it should haven been "in der Nacht". Please explain it for me.

  • There's a small difference in meanings. "Ich arbeite bis spät in der Nacht" means "I often work until late in the night". You have a sense of direction there, as Hubert Schölnast explains in his answer. It is implied that the work started (well) before nightfall. "Ich arbeite oft spät in der Nacht" (note that there's no "bis" here) means "I often work late at night". This version doesn't imply when the work started. It only notes the place (in time) where the work takes place. But as guidot said, "Wechselpräposition" is the keyword to read up on here. – Henning Kockerbeck Nov 10 '20 at 8:50

In German you always make a strong difference between a place and a direction. The place gets dative case, the direction accusative case. And this concept also holds for temporal "places" (moments) and temporal "directions".

When you work until some moment, then it is the same as if you move to a certain place. Both is a direction and therefore needs accusative case.

  • temporal place

    I am working in the night.
    Ich arbeite in der Nacht. ← Dative case

  • temporal direction

    I am working until the night.
    Ich arbeite bis in die Nacht. ← Accusative case

This concept of places and direction also holds for modal places and directions:

Ich bin heute nicht in der richtigen Stimmung. ← Modal "place" = dative case
I am not in the right mood today.
Ich komme heute nicht in die richtige Stimmung. ← Modal "direction" = accusative case
I can't get into the right mood today.

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