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?