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Большое спасибо за помощь!

I know Я не мог means "i couln't" ,

is it ever possible to use Я мог не ... in the sense of "i didn't have to" or "i was allowed not to"?

Are these the most common ways to say the follwing in Russian?:

you must not do that.
(Вам не надо это делать)

you don't have to do that.
(Вы не обязаны это делать ; вы могли бы это не...