Why is 'satte sig ned' wrong??
Two years later and after all these questions still no answer why "satte sig ned på" is incorrect...
"ned" is not really necessary, because "at sætte sig" is the action of sitting yourself down.
That said, it wouldn't be wrong to use either, but it hasn't been written in as an alternative.
No ‘ned’ in this very strange sentence. Who sits on anyone’s head anyway. Very uncomfortable and rude. Unless it’s a toy thing perhaps it doesn’t make much sense.