Learn Finnish: Näitkö sä, mitä sit sattui? - Did you see what happened then?

Sä is the very common spoken language (puhekieli) version of sinä (you). In everyday casual speech, Finns almost always use sä instead of the formal written sinä.

Is sit just a shortened version of sitten?

Yes! As you are learning in this lesson, everyday spoken Finnish often drops letters from common function words. Sitten (then) almost always becomes sit or sitte in normal conversation.

Doesn't the verb sattua usually mean "to hurt"?

It does mean "to hurt" (as in se sattuu - it hurts), but it is also a very common way to say "to happen" or "to occur by chance". Here, mitä sit sattui simply means "what happened then".

Why does the sentence use mitä instead of mikä for "what"?

Mitä is the partitive form of mikä. When referring to a general, abstract, or undefined event like "what happened", Finnish uses the partitive mitä. You would use mikä if you were asking to identify a specific, concrete thing.