Did you know that the Russian prepositions of place под and за can be used both with the Accusative and the Instrumental case, depending on their meaning? If they mean a direction, they are used with the Accusative case, if they mean a place, they are used in the Instrumental case. The prepositions между, перед, под and над are only used with the Instrumental case, both in the meaning of place and direction.
Мяч лежит под диваном. The ball in under the table.
Положи мяч под диван. Put the ball under the table.
За тарелкой стояла чашка. The cup was behind the plate.
Поставь чашку за тарелку. Put the cup behind the plate.
Перед нами была картина. There was a painting in front of us. (Place, Instrumental).
Повесь картину перед нами. Hang the picture in front of us. (Direction, Instrumental).