When do we use "Czemu" as opposed to "Dlaczego" for 'Why'?
Czemu is colloquial, Dlaczego is official. There are no differences in meaning.
How colloquial is it? Is it accepted enough to use in writing? For instance, is it more formal than 'gonna' or 'wanna' in English in that you can write them online, but you would never put them in anything more formal than online chat with friends.
could you add why are you all so tired? It makes sense because of the "jesteście" Thanks :)
"How come you're so tired?"