@swamp,
Quote:It was simply impossible to anyone of us to get into the camp because none of us knew the password.
According to the answer,it tells me I should replace the anyone to any one?but why?I think the anyone is reasonable.
My best guess is that is has to do with the structure of the sentence and internal consistency. "none of us" is different from "no one" because the second would include everybody whereas the first would be limited to none of a specific group.
Likewise, it's not impossible for "anyone" (i.e, everyone in this usage) to get in. It's just impossible for any "one" of a limited number (a specific group).
Apart from the grammar police, though, I don't think anyone would care or be confused by either usage (anyone or any one).