一、情态动词表推测的三种形式1.情态动词+动词原形，表示对现在某动作或状态的推测，比如must do，can't be等。例：It can't be true.这不可能是真的。2.情态动词+动词进行式，表示推测某动作可能正在进行，比如must be doing等。例：Lily must be doing her homework now.莉莉现在一定是在做作业。3.情态动词+动词完成式，表示对过去行为或动作的推测，比如must have done等。例：He must have misunderstood me.他一定是误解了我。
1.must have done：推测某事必定已经发生，只用于肯定句，否定形式用can't have done。例：The light is on.Father must have come back.灯亮着，爸爸一定已经回来了。
2.could have done：对已发生的事的可能性进行推测。例：The dictionary has disappeared.Who could have taken it?字典不见了。会是谁拿了呢?could have done还可以表示某事本来可能发生却未发生。例：I could have slept for long,but Ididn't.我本来可以睡的时间长一点儿，但是我没有。
3.can't/couldn't have done：表示某事不可能已经发生，couldn't have done语气更委婉。例：She can't have gone to school.It's Sunday.她不可能去上学了。今天是星期天。
4.may/might have done：推测某事可能已经发生，might have done所表示的可能性更小。例：Polly is very late.She may have missed her train.波莉到现在还没有来。她可能是误了火车。
5.should/ought to have done：表示某事本?该发生却未发生，含有责备、遗憾的意味。例：She should have arrived here by now.她现在应该已经到这儿才对啊。6.shouldn't/oughtn't to have done：表示某事本不应该发生却已经发生，含有责备的意味。例：You oughtn't to have spoken to them in that way.你不应该以那种方式对他们讲话。7.need have done：表示过去有必要做却没有做，含有责备的意味。例：You need have practiced your spoken English more often.你本来应该多练习英语口语的。8.needn't have done：表示做了不必要做的事。例：There was plenty of time.She needn't have hurried.那会儿时间还很多，她没必要那样匆忙。
表推测时，英语中只使用must,may,might,may not和can't。这五个表达的语气依次递减：
must:一定（语气肯定）
may：也许（不很肯定）
might：或许（比may语气更弱）
may not：也许不（表否定）
can't：一定不（must的反义）
推测现在的事情用must,may,might,may not和can't+do或be
e.g.She must be at home now.
The boy may play now.
推测过去的事情用must,may,might,may not和can't+have done
e.g.She might have been ill yesterday.
He must have hold the party.
注意：表推测的must,may,might,may not和can't是不可以与I think...连用。因为I think...已有推测的含义，故不能连用。
