- For Teachers
You ___________ have handed the work in today. The deadline has been extended until next week.
Choose one answer.
I think it is shouldn't but not sure
I can't support "shouldn't" as a good answer, even though "needn't" isn't used much in the US. (We would be more likely to say "You didn't have to hand in..."
But "shouldn't" implies that there was something WRONG with turning it in, instead of simply being unnecessary.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
"Needn't" is the only one that works for me if we assume that the question simply dealt with a standard deadline for something like handing in a college essay. Most people don't hand in a piece of work until they need to (ie until there is almost no time left before the deadline).
As Barb said, "shouldn't" suggests that you have done something wrong which isn't the case.
"Can't" would express disbelief. "You can't have handed it in today. You hadn't even started it last night!"
"You mustn't have handed it in ..." simply isn't natural English, no matter what the context is.