German prepositions always force a specific case onto the noun that follows them. Some always take the dative, some always take the accusative, and nine "two-way" prepositions can take either, depending on whether the sentence describes a location or a direction. This lesson mixes all three groups together — the real skill isn't applying a known rule, it's recognizing which rule applies.

Always dative: aus, bei, mit, nach, seit, von, zu

Always accusative: durch, für, gegen, ohne, um

Two-way (dative or accusative): an, auf, hinter, in, neben, über, unter, vor, zwischen

• Wo? (where — a fixed location, nothing is moving) → Dativ. Typical verbs: sein, liegen, stehen, sitzen, hängen (intransitive).
• Wohin? (where to — movement into or onto a place) → Akkusativ. Typical verbs: legen, stellen, setzen, hängen (transitive), or gehen/laufen/fahren + preposition.

Compare: Das Buch liegt auf dem Tisch (it's already there — dative) vs. Ich lege das Buch auf den Tisch (I'm putting it there — accusative, motion happening). Watch for these look-alike verb pairs that differ in motion: liegen (lie/be lying, dative) vs. legen (lay/put, accusative); stehen (stand, dative) vs. stellen (put upright, accusative); sitzen (sit, dative) vs. setzen (set/place, accusative).

In everyday speech, dative contractions are common: bei dem → beim, von dem → vom, zu dem → zum, zu der → zur, in dem → im, an dem → am. This lesson uses the uncontracted forms so the case stays visible.