to = this will affect you (a bit negative) for = more beneficial But I don't think that "something happens for you" is very likely, as the sentence is rather passive and implies that there is no intention behind the result. For example the sun rises every day, but it doesn't do that for anybody. It just does. (happens) So, use "for" in sentences like "I'm doing this for you." or "He participated for the sake of raising public awareness."

Depends what you want to say. Both are fine.

This will happen to you = you will receive the action (whether you want it or not). This will happen for you = you will benefit from it.

to.........(for that sentence, I dont know why but it heard more suitable)

to