1.省略连词if。在书面语中，如果虚拟条件从句中有were，had 或 should，可以把if省略，把这几个词放到主语之前，构成主谓倒装。

　Should he come (If he should come), tell him to ring me up. 他要是来了，让他给我打个电话。

Were I you (If I were you), I would not do it. 我要是你，就不做这事。

2.虚拟与陈述的混合

He could have passed the exam, but he wasn’t careful enough.

You should have come earlier. The bus left a moment ago.

(三)特殊句式虚拟结构

1.不同时间的虚拟

If he had listened to me, he would not be in trouble now.(从句与过去事实相反，主句与现在事实相反)

If he had told me yesterday, I should know what to do now. (从句与过去事实相反，主句与现在事实相反)

If I were you, I would have gone to her birthday party. (从句与现在事实相反，主句与过去事实相反)

If you hadn’t lent me some money, I couldn’t have bought the new house and most likely I would be still living in the dangerous house now. (从句与过去事实相反，and后面的主句与现在事实相反)

(一)一般虚拟结构　　

注意:主句中的should只用于I、we，但在美国英语中，should常被would代替；从句中的should可用于各种人称，且不可以被would所代替。

(二)混合虚拟结构

11.其它情态动词

　 　①have to

have to表示 “必须, 不得不”, 在这个意义上与must很接近, 但must表示的是说话人的主观看法, 而

have to表示的却是客观需要. have to比must有更多的形式。

The TV set is broken. I have to buy a new one.

The students will have to know how to use the computers.

注意：在回答must的问句时, 否定式常用need not(needn’t)或don’t have to表示 “不必”。而不用

must not, 因为must not表示 “不可以”。

　 ②used to

表示过去的习惯动作或状态(现在已不复如此)

-Did you use to go there to see your brother? /Used you to go there to see your brother?

-Yes, I did(used to). /No , I didn’t (usedn’t).

注意：would也可表示过去反复发生的动作或某种倾向，但是不涉及到现在。

③had better

表示 “最好(做……)”

We had better go now.　

　　 其否定式为had better not

10.情态动词+have done的用法

①could+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.

②cannot+have done：表示对过去行为的否定推测

　　 He cannot have been to that town.

③can+主语+have done：表示对过去行为的怀疑或不肯定

Can he have got the book?

④might(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.

(Mood:might>may,possibility:might<may)

⑤must+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.

⑥needn’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.

⑦should(ought to)+ have done：本来应该做而实际上又没有做。

其否定形式表示某种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 wanted to use it.

　　 注意：本结构还可以表“推测”的意思

He should have finished the work by now.

到现在他应该已经做完那项工作了。

9.dare和need

①need作为情态动词只有一种形式，只用于否定句和疑问句。

②dare作为情态动词用时有两中形式：dare和dared两个词形。

　　 除了可以用于否定句和疑问句外，还可以用于条件从句或表示怀疑的句子中。

　　 If he dare come,I will kick him out.

　　 I don’t know whether he dare say.

Note:He doesn’t dare(to)answer the question.

　　 Does she dare(to)enter the dark room?

8.will/would

①请求、建议，would比will委婉客气

　　 Would you pass me the book?

②表示意志、愿望和决心

　　 I will never do that again.

　　 They asked us if we would do that again

③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.

④表示估计或猜想

　　 It would be about ten when he left home.

　　 What would she be doing there?

7.shall/should

What shall we do this evening?

注意：may用于征求对方的许可，而shall用于征求对方的意见或指示：

May I have a look?我能看一看吗？

Shall I have a look?需要我看一看吗？

You shall fail if you don’t work harder. (警告)

He shall have the book when I finish reading. (允诺)

He shall be punished.(威胁)

The fine shall be given in cash.罚款须以现金缴纳。

①You should apologize to him.

②The photos should be ready by 12:00.

③I’m surprised that you should speak in such a way.

④Who should ccome in but my old friend Betty.我当究竟是谁进来了呢，原来是我的老朋友贝蒂呀。

⑤I suggested that he (should)change his mind.

My suggestion was that he (should)change his mind.

注意：ought to表示 “应该” (与should同义, 只是语气稍重一些), 也可表示推测。

You ought to take care of him.　　　

He ought to be home by now.

6.表示祝愿、祝福的may,此时句子用倒装语序

　 May he live to an old age!愿他长命百岁！

May God be with you!愿上帝保佑你！