If you ever read any Agatha Christie, you may notice that occasionally Poirot gets overly excited and says things like, "But what passes itself here?" That "passes itself" is about the best direct translation you can make from "se passer." Of course it sounds silly in English, like that.
I was quite pleased with myself when I was able to fluently say "qu'est qui s'est passé" without having to spend five minutes getting it straight in my head first. Now there's this alternate formulation! Can anyone tell if the are completely equivalent or when you might use one instead of the other?
It's just the inversion method of forming a question, which you probably know if you stop to think about it.
The basic statement is Il s'est passé. = "It happened." Using inversion (s'est-il) and a question word (que = what) the question you get is
- Que s'est-il passé ?
The two forms have equivalent Meanings of "what happened?" This inversion form is more often used in writing. The qu'est-ce qui form is used when speaking, or writing a little less formally.