should 劝告、建议、命令、应该做、道义上的责任。 You should(ought to) go to class right away.

I should(ought to) help him because he is in trouble. will/would 请求、建议，would比will委婉客气。 Would you pass me the book? 表示意志、愿望和决心。 I will never do that again.

They asked us if we would do that again would可表示过去反复发生的动作或某种倾向。 During the vacation he would visit me every week

The wound would not heal.（伤口老是不能愈合） would表示估计或猜想。 It would be about ten when he left home.

What would she be doing there? 情态动词+have done的用法 could+have done：本可以做而实际上未能做。 You could have done better, but you didn't try your best. cannot+ have done：表示对现在或过去行为的否定推测。 He cannot have been to that town. can+主语+ have done：表示对过去行为的怀疑或不肯定（用在疑问句中）。 Can he have got the book? might(may)+ have done：对过去发生的行为不太肯定的推测。 He may not have finished the work.

If we had taken the other road, we might have arrived earlier. must+ have done：对过去发生的行为肯定的推测。

其否定式为：cannot have done。 You must have seen the film.

You cannot have seen the film. needn't+ have done：本来不必要做的而实际上又做了。 You needn't have watered the flowers,for it is going to rain.

注意：didn't need to(have to)do：

没有必要做而实际上也没有做

I didn't need to clean the windows.My sister did it 2 hours ago. should(ought to)+ have done：本来应该做而实际上又没有做。其否定形式表示某中行为不该发生却发生了。 You should have started earlier, but you didn't.

She shouldn't have taken away my measuring tape,

for I worked to use it.

注意：He should have finished the work by now.（表推测） 　虚拟语气

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