can't是不可能的意思。例:I can't move that box because it is so heavy.
don't是
助动词。例:I don't know.
did't是don't的
过去式,在一般过去式的句子中改否定句是常用到。例:He worked out this problem by himself.的否定句是He didn't work out this problem by himself.
couldn't是can't的过去式。could也有时候表示委婉语气。例:Could you give me that pen?
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