I can't understand predicate

I can't quite understand predicate, I tried to learn it by doing the basic sneaking detection, it worked. Now I wanted to make something by myself. basically, if an entity hit something with a blase rod in it's hand, I want to give slowness 4 for 2 second on the targeted entity. For now i tried a basic check. the command block tellraw to all player with the predicate as true "you are stun!" But it doesn't work. I don't understand if it is my rudimentary "check" or just the predicate itself. Im lost, I read the wiki 2 or 3 times in a row... I guess i don't understand how predicate work