N-, mag-/si-, ma-, magsi-
To put weight on s.t. (as to hold s.t. down; as when sleeping on one's shoulder; as when accidently rolling over in one's sleep on top of s.o., etc.).
The pa--un imp. focuses on the object that you put on top of s.t. to pin it down. The -an imp. focuses on the object that is being pinned down. When the ta- prefix is used the -an seems to be dropped when it means happened to. But when it means able to then the -an is added.
Put s.t. on top of that paper so that the wind doesn't blow it away.
My arm is numb from having slept on it all night.