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4.can could may must后接完成式的用法 (1)can could后接完成式的用法 在否定式或疑问句中,表示对过去发生过的事情的怀疑或不肯定的态度。如:Could he have said so?
在肯定句中,可以表示过去可能做到而实际并没做到的事情,有劝告或责备的语气。如:--When did you answer her letter?--Only yesterday.--It's too late.You could have answered it earlier,I am sure.


来自手机贴吧110楼2013-01-31 11:22
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    have to的基本用法
    have to和must的意义相近,只是must侧重表示说话人的主观看法,而have to则表示客观需要。如:I must study hard.
    I had to give it up because of illness.


    来自手机贴吧112楼2013-01-31 11:33
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