"The fish is in the net."
Translation:Der Fisch ist im Netz.
The preposition "in" can be dative or accusative depending on the context. Accustive=action, so if the fish was in the process of getting put in the net, it'd be accusative (The fish goes in the net= accusative). Since the fish is already in the net and there is no action involved, it is dative (The fish is in the net= dative).