"You are lying on me."
Translation:Du liegst auf mir.
But you are lying on me may also mean an action in progress. So wouldnt "auf mich" also be used?
The accusative has nothing to do with action versus inaction.
After a two-way preposition, the accusative is used to describe the destination of a motion.
"lying" is not motion, and you can't "lie onto someone", so auf mich makes no sense here.
That means something completely different -- it means, "You are lying to me" (i.e. you are not telling the truth to me; you are saying something to me that is not correct) rather than "You are lying on me" (i.e. you are horizontal and are above me such that your body is resting on mine horizontally).
"lie" is ambiguous in English between "tell an untruth" and "rest horizontally", but in German, lügen and liegen are similar but distinct.
You would use "auf" in the sense that something is on something else. (e.g. The cat is on the sofa. = Die Katze ist auf dem Sofa.) "An" can either mean at or on as in to turn something on. (e.g. I am at the table. = Ich bin an dem Tisch. OR Mach das licht an! = Turn on the light!)
Hope this helps :)
the English sentence does not make sense like that (it should have been "You are lying to me" then)
Syfogidas was trying to construct a situation along the lines of "you are on top of me and you are lying = you are lying on me".
I also suspect the German should be "gegen mir"
German uses a separate word for lying to someone: jemanden anlügen. So "you are lying to me" is du lügst mich an.