In this sentence, how do we know (apart from the '?') if it is a statement or a question, without 'không' at the end?
Because of ừ. This implies a question when you are already pretty certain of the answer like a tag question, adding ',right?' or 'eh?' In English. E.g. It's pretty cold today, eh?
Yes, why is không or ư being marked wrong here?
you can say " không ai thông báo cho bạn à?. "ư" i think it is ok. someplace still say like that.
Accepts "...thông báo với bạn"