So far, you have studied prepositions that always take the dative case (bei, mit, von, etc.) or the accusative (durch, bis, fur, etc.). Now, look at those that can take either case, depending on the context. These two-way" prepositions include an, auf, and in. Study the verbs that often accompany them, expressing either location (and, therefore, dative) or placement/destination (hence accusative)."