How do I make an object (in modmaker or in notepad) able to do damage to your opponent but not to you?

I think it's not possible, you would need to divide the positions and create two objects that make damage.

Maybe a script..

well as far as I know all obj damage both players. You need to use the trigger sistems in order to make an obj hurt only one player.

Is there a tutorial on this (trigger systems). I can only find the ones that trigger wins/loses.