can someone help understand why this sentence is correct:
It's impossible - they mustn't have finished it already!
It's impossible - they can't have finished it already!
As a Brit, but not a teacher:
I'd need to know more of the context to reply properly, but, on the information provided so far, I would agree that the 2nd sentence is the more likely one that I would use.
The 1st sentence suggests that they are not allowed to finish yet, whereas the 2nd sentence suggests that they couldn't possibly have finished yet, as, say, you would have expected them to have spent much more time just physically responding to the question, without taking into account the time to think up an answer!
Hope this helps