To go or come after s.o.; to follow; to walk behind s.o.
The pa--an imp. focuses on the thing that is having s.t. follow after such as a pill that is having a glass of water follow after it in order to be swallowed. The pa--un imp. focuses on the thing that is being caused to follow after s.t. else. The -un imp. focuses on the person or animal that is being followed.
If you want to eat a pastry, go to where your father is at the market.