"Are you drinking my coffee out of my cup?"
Translation:Trinken Sie meinen Kaffee aus meiner Tasse?
I believe that that's because the coffee is the direct object, so that should be in the accusative case. The cup is the indirect object, so that should be in the dative case. Mein becomes meinen in the accusative case if the noun is masculine (der Kaffee), and meiner in the dative case if the noun is feminine (die Tasse). Hopefully somebody can correct me if I'm wrong!