This is a question from the B2 Russian exam.
Я очень рад: домашнее задание сегодня можно не а)сдавать б)сдать
The correct answer is listed as (a). Why? It's a one-time action, and doesn't have any continuous quality. Any help would be appreciated.
Можно не сдавать — (we) don't have to submit the homework today
It is possible and/or allowed (можно) for us to not not submit it, i.e., not do the action at all.
This is just normal use of imperfective. That is, not doing the action at all.
Можно не cдать — means we might not be able to get it handed in
It is possible that we cannot (won't be able to) hand it in.
Он может не приходить— He can not come. (if he wants) = He doesn't have to come.
Он может не прийти —He might not be able to come. (because something might happen)
This is just normal perfective. There is a chance that someone won't be able to finish, accomplish, or get something done.
This is grammar structure in Russian that corresponds roughly to— 1) don't have to, and 2) might not be able to.
I think, in case with сдать/сдавать in context of an exam, homework, etc., "сдать" is used not as "to submit answers" but to "pass an exam" (get a good score for homework", etc.), while "сдавать" is used for the process of passing an exam or submitting a homework.
Short answer: we change a perfect verb (сдать) to its non perfect form (сдавать) in negative sentences (можно не сдавать) with a modal verb (можно).
There's a more detailed explanation here (go to section 6.2.3)