Why are the formats for questions different for all the questions? If I had put this format of a question for another sentence it would have been marked wrong, very annoying.
It's about context, I think. The question above isn't really a question because the speaker has already known that "she" in the sentence can speak Hebrew. That's why you can't use it in other question format. I think.
"She knows Hebrew? that's not possible!" was accepted, I think "not possible" should be accepted. I mean, how do you otherwise say it (not possible), לא אפשרי?
i think that should be accepted as well.
For the record, not possible is אי אפשר pronounced "ee efshar"
בלתי אפשרי more literally means "non possible," ie "impossible" and is an adjective, whereas אפשר is an adverb
however, in English we could say "not possible" or "impossible" and it would be the same. I am not sure if these are interchangeable or not in Hebrew. I suppose technically אי אפשר would reference a verb, and בלתי אפשרי would reference a concept/noun.
If it simply said "בלתי אפשרי" then "not possible" would be accepted. But because it says "זה בלתי אפשרי", "not possible" is not accepted because it fails to translate זה.
Just for clarification though, for a person it would not be interchangeable in English, as impossible would mean hard to handle or a pain in the ****. One would not generally say "you are not possible."