(一)在主语从句中
1. 用于It is advisable/ odd/ desirable/ imperative/ natural/ natural/ necessary/ strange/ unusual, etc. + that 引导的主语从句中。
It is highly desirable that a new president (should) be appointed for the college.
2. 用于It is desired, it is suggested, etc. +that 引导的主语从句中。
It is urged that he(should) be trained.
It is requested that Mr.Wang(should)give a performance at the party.
(二)虚拟语气用于下列动词后的宾语从句中
advise arrange move(提议) beg demand deserve
desire determine recommend ask insist order
prefer propose requite wish urge suggest
suppose decide
I asked that I (should) be allowed to read whatever I liked.
She move/ suggested that the meeting (should) be continued after lunch.
如果宾语从句中的动词是否定的，从句后应是not + 动词原形。
The doctor suggested that he not smoke.
(三) 除了wish表愿望外，还可以用在would rather, had better, would as soon 结构的宾语从句中。
I would rather you come tomorrow.
I would rather you came today.
I would rather you had come yesterday.
I would just as soon you didn’t speak rudely to her.
(四) 虚拟语气用在条件状语从句中
1.当非真实条件句中谓语动词含有 should, had 或 were 时，有时可以省略if，采用倒装结构，即将should, had 或 were 移到主语前面。如：
Should you change your mind, let me know.
Were there no gravity, there would be no air around the earth.
Had they acted sooner, the strike wouldn’t have happened.
2. 有时主句和从句中的动词形式不一致。比如主句为 should/would/could + 动词原形(指现在)，而从句为had + 动词的过去分词(指过去);从句为 should + 动词原形，主句用陈述语气或祈使句。这称作错综虚拟语气。
You would be happier now if you had taken his advice.
If you were in better health, we would have allowed you to go boating with us.
3. 有时句子中未出现条件句，而是由一些短语提供假设的情况。
He would have come to your party, but he was busy.
With his help, you would have succeeded in it.
But for the rain, we should have had a pleasant journey.
Without your advice, I couldn’t have done it so quickly.
4. if it were not for 表示与现在或将来相反，If it hadn’t been for表示过去事实相反。
Were it not for the air, all plants should die.
If it hadn’t been for your help, I really don’t know what I’d have done.
5.虚拟语气用于It’s(high, about)time + that 的定语从句中，表示现在和将来，不表过去。that 从句中的谓语应用过去时(be 常用were或should/had better/had rather + 动词原形)，有时可用一般时或进行时表示委婉的建议、命令等。
It’s high time that he washed his dress.
I’m getting tired – It’s time we went to bed.
It’s about time we were going.
6. 用于 if only 含有“但原(愿)”，“要是…就好了”。其用法wish基本相同。
If only he know(had known) how we missed him!
If only he were alive.
If only I hadn’t made such a mistake!
7. 用于otherwise 或or 引导的并列句中。
I don’t know where he is, otherwise(or) I would telephone him.
8. 用在lest, in order that, in case引导的目的状语从句中。
He told his wife as little as possible, lest she (should) spread the news all over town.
9. 用在as if, as though引导的方式状语从句中。
She spoke to me as if I were deaf.(表同时或之后发生)
She behaved as if nothing had happened.(表从句动作在主句动作之前发生)
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