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can't与couldn't的区别

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can't与couldn't的区别
can't与couldn't的区别
can't/couldn't表示推测时,意思是“不可能...”,语气非常坚决的否定推测.
根据动作发生的时间,一般分为以下三种情况:
1)进行式(对正在发生的事情的否定推测):can't be doing sth
如:It's already ten o'clock.My father can't be sleeping now.
2)完成式(对过去的动作的否定推测):can't/couldn't have done sth
如:He is at schoool,so he can't/couldn't have stayed up last night.
3)一般式(对现在/将来情况的否定推测):can't do sth
如:Your answer can't be true.
can/could表示推测时一般只用于否定句/疑问句,但是也可用于肯定句,只是比较少见.
其中could不表示过去式,而表示可能性较小.
如:I think the work can be completed ahead of time.我认为这项工作能提前完成.