（一）在主語(yǔ)從句中

1. 用于It is advisable/ odd/ desirable/ imperative/ natural/ natural/ necessary/ strange/ unusual, etc. + that 引導(dǎo)的主語(yǔ)從句中。
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 引導(dǎo)的主語(yǔ)從句中。
It is urged that he(should) be trained.
It is requested that Mr.Wang(should)give a performance at the party.
（二）虛擬語(yǔ)氣用于下列動(dòng)詞后的賓語(yǔ)從句中

advise               arrange             move(提議)     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.
如果賓語(yǔ)從句中的動(dòng)詞是否定的，從句后應(yīng)是not + 動(dòng)詞原形。
The doctor suggested that he not smoke.
(三) 除了wish表愿望外，還可以用在would rather, had better, would as soon 結(jié)構(gòu)的賓語(yǔ)從句中。
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) 虛擬語(yǔ)氣用在條件狀語(yǔ)從句中
1.當(dāng)非真實(shí)條件句中謂語(yǔ)動(dòng)詞含有 should, had 或 were 時(shí)，有時(shí)可以省略if，采用倒裝結(jié)構(gòu)，即將should, had 或 were 移到主語(yǔ)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. 有時(shí)主句和從句中的動(dòng)詞形式不一致。比如主句為 should/would/could + 動(dòng)詞原形（指現(xiàn)在），而從句為had + 動(dòng)詞的過(guò)去分詞（指過(guò)去）；從句為 should + 動(dòng)詞原形，主句用陳述語(yǔ)氣或祈使句。這稱(chēng)作錯(cuò)綜虛擬語(yǔ)氣。
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. 有時(shí)句子中未出現(xiàn)條件句，而是由一些短語(yǔ)提供假設(shè)的情況。
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 表示與現(xiàn)在或?qū)��?lái)相反，If it hadn’t been for表示過(guò)去事實(shí)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.虛擬語(yǔ)氣用于It’s(high, about)time + that 的定語(yǔ)從句中，表示現(xiàn)在和將來(lái)，不表過(guò)去。that 從句中的謂語(yǔ)應(yīng)用過(guò)去時(shí)（be 常用were或should/had better/had rather + 動(dòng)詞原形），有時(shí)可用一般時(shí)或進(jìn)行時(shí)表示委婉的建議、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 引導(dǎo)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引導(dǎo)的目的狀語(yǔ)從句中。
He told his wife as little as possible, lest she (should) spread the news all over town.
9. 用在as if, as though引導(dǎo)的方式狀語(yǔ)從句中。
She spoke to me as if I were deaf.(表同時(shí)或之后發(fā)生)
She behaved as if nothing had happened.(表從句動(dòng)作在主句動(dòng)作之前發(fā)生)