（一）在主語從句中

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

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.
如果賓語從句中的動詞是否定的，從句后應是not + 動詞原形。
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 結構的賓語從句中。
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) 虛擬語氣用在條件狀語從句中
1.當非真實條件句中謂語動詞含有 should, had 或 were 時，有時可以省略if，采用倒裝結構，即將should, had 或 were 移到主語前面。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. 有時主句和從句中的動詞形式不一致。比如主句為 should/would/could + 動詞原形（指現(xiàn)在），而從句為had + 動詞的過去分詞（指過去）；從句為 should + 動詞原形，主句用陳述語氣或祈使句。這稱作錯綜虛擬語氣。
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. 有時句子中未出現(xiàn)條件句，而是由一些短語提供假設的情況。
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)在或將來相反，If it hadn’t been for表示過去事實相反。
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.虛擬語氣用于It’s(high, about)time + that 的定語從句中，表示現(xiàn)在和將來，不表過去。that 從句中的謂語應用過去時（be 常用were或should/had better/had rather + 動詞原形），有時可用一般時或進行時表示委婉的建議、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引導的目的狀語從句中。
He told his wife as little as possible, lest she (should) spread the news all over town.
9. 用在as if, as though引導的方式狀語從句中。
She spoke to me as if I were deaf.(表同時或之后發(fā)生)
She behaved as if nothing had happened.(表從句動作在主句動作之前發(fā)生)