不用 needn't, don't need
不能 can't, couldn't
不要 don't, shouldn't
如:
你不用说了 You needn't say anying. / You don't need to say any more.
你不能再说了 You can't say any more!
你不要再说了 Shut up, OK? (一般口语中这么说比较自然,而不用 you don't say more. 但不要的本意是 don't , shouldn't)
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