Quote Originally Posted by krwright View Post
Does this not require the instrumental? I read somewhere that за meaning 'behind' takes instrumental. Maybe I am wrong?
Consider this a difference the same as between "in" and "into", "at" and "to". Just like "на" and "в" require Prepositional for expressing a location and Accusative for describing a detination of movement, "за" takes a noun in Accusative when you are describing motion with a goal to get behind something. When you are simply telling the position behind some object, it takes Instrumental. Even when the position is not itself static Eg. "За мной!" which can be used as "follow me" or "иди за мной"... doesn't matter much when used as a command).