Two-way prepositions

Nine prepositions can take either case: an, auf, hinter, in, neben, über, unter, vor, zwischen. The rule is about movement: if you're asking wo? (where — location), use the dative. If you're asking wohin? (where to — direction), use the accusative.

Ich bin im Kino (dative: I'm at the cinema). Ich gehe ins Kino (accusative: I'm going to the cinema). The verbs of placing and being placed help: legen/liegen, stellen/stehen, hängen.