1） 概念
虛擬語氣用來表示說話人的主觀願望或假想，所說的是一個條件，不一定是事實，或與事實相反。
2） 在條件句中的應用
條件句可分為兩類，一類為真實條件句，一類為非真實條件句。非真實條件句表示的是假設的或實際可能性不大的情況，故採用虛擬語氣。
真實條件句
真實條件句用於陳述語氣，假設的情況可能發生，其中 if 是如果的意思。
時態關係
句型： 條件從句主句， 一般現在時 shall/will + 動詞原形
If he comes, he will bring his violin.
典型例題
The volleyball match will be put off if it ___.
A.will rainB. rainsC. rainedD. is rained
答案B。真實條件句主句為將來時，從句用一般現在時。
注意：
1） 在真實條件句中，主句不能用be going to表示將來，該用shall, will.
(錯) If you leave now, you are never going to regret it.
(對) If you leave now, you will never regret it.
2） 表示真理時，主句謂語動詞便不用shall (will) +動詞原形，而直接用一般現在時的動詞形式。
非真實條件句
1）時態：可以表示過去，現在和將來的情況。它的基本特點是時態退後。
a.同現在事實相反的假設。
句型 ： 條件從句主句
一般過去時should( would) +動詞原形
If they were here, they would help you.
b.表示於過去事實相反的假設。
句型： 條件從句主句
過去完成時should(would) have+ 過去分詞
If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
If he had come yesterday, I should / would have told him about it.
含義：He did not come yesterday, so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress.
含義： He was ill and missed many lessons, so he did not make greater progress.
c.表示對將來的假想
句型：條件從句 主句
一般過去時should+ 動詞原形
were+ 不定式 would + 動詞原形
should+ 動詞原形
If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
混合條件句
主句與從句的動作發生在不同的時間，這時主，從句謂語動詞的虛擬語氣形式因時間不同而不同，這叫做混合條件句。
If you had asked him yesterday, you would know what to do now.
（從句與過去事實相反，主句與現在事實相反。）
If it had rained last night (過去), it would be very cold today (現在).
虛擬條件句的倒裝
虛擬條件句的從句部分如果含有were, should, 或had, 可將if省略，再把were, should或had 移到從句句首，實行倒裝。
Were they here now, they could help us.
=If they were here now, they could help us.
Had you come earlier, you would have met him
=If you had come earlier, you would have met him.
Should it rain, the crops would be saved.
=Were it to rain, the crops would be saved.
注意：
在虛擬語氣的從句中，動詞'be'的過去時態一律用"were"，不用was， 即在從句中be用were代替。
If I were you, I would go to look for him.
如果我是你，就會去找他。
If he were here, everything would be all right.
如果他在這兒，一切都會好的。
典型例題
_____ to do the work, I should do it some other day.
A. If were I B. I wereC. Were ID. Was I
答案C. 在虛擬條件狀語中如果有were, should, had這三個詞，通常將if省略，主語提前, 變成 were, should, had +主語的形式。但要注意，在虛擬條件狀語從句中，省略連詞的倒裝形式的句首不能用動詞的縮略形式。如我們可說 Were I not to do., 而不能說 Weren't I to do.
特殊的虛擬語氣詞：should
1） It is demanded / necessary / a pity + that…結構中的主語從句的謂語動詞要用should 加動詞原形， should 可省略。
句型：
(1)suggested
It is(2)importantthat…+ (should) do
(3) a pity
(1)suggested, ordered, proposed, required,demanded, requested, insisted； + (should) do
(2)important, necessary, natural, strange
a pity,a shame,no wonder
(3)It is suggested that we (should) hold a meeting next week.
It is necessary that he (should) come to our meeting tomorrow.
2）在賓語從句中的應用
在表示命令、建議、要求等一類動詞後面的從句中。
order, suggest, propose, require, demand, request, insist, command, insist + (should) do
I suggest that we (should) hold a meeting next week.
He insisted that he (should ) be sent there.
注意： 如suggest, insist不表示"建議" 或"堅持要某人做某事時"，即它們用於其本意"暗示、表明"、"堅持認為"時，賓語從句用陳述語氣。
The guard at gate insisted that everybody obey the rules.
判斷改錯:
（錯）You pale face suggests that you (should) be ill.
(對)Your pale face suggests that you are ill.
(錯)I insisted that you ( should) be wrong.
(對)I insisted that you were wrong.
3）在表語從句，同位語從句中的應用
在suggestion, proposal, idea, plan, order, advice等名詞後面的表語從句、同位語從句中要用虛擬語氣，即（should）+動詞原形。
My idea is that we (should) get more people to attend the conference.
I make a proposal that we (should) hold a meeting next week.
wish的用法
1）用於wish後面的從句，表示與事實相反的情況，或表示將來不太可能實現的願望。其賓語從句的動詞形式為：
真實狀況 wish後
從句動作先於主句動詞動作
（be的過去式為 were） 現在時 過去時
從句動作與主句動作同時發生
（had + 過去分詞） 過去時 過去完成時
將來不大可能實現的願望 將來時 would/could +動詞原形
I wish I were as tall as you.我希望和你一樣高。
He wished he hadn't said that. 他希望他沒講那樣的話。
I wish it would rain tomorrow. 我希望明天下雨就好了。
2）Wish to do表達法。
Wish sb / sth to do
I wish to see the manager. = I want to see the manager.
I wish the manager to be informed at once. (= I want the manager to be informed at once.)
比較if only與only if
only if表示"只有"；if only則表示"如果……就好了"。If only也可用於陳述語氣。
I wake up only if the alarm clock rings. 只有鬧鐘響了，我才會醒。
If only the alarm clock had rung.當時鬧鐘響了，就好了。
If only he comes early.但願他早點回來。
It is (high) time that
It is (high) time that 後面的從句謂語動詞要用過去式或用should加動詞原形，但should不可省略。
It is time that the children went to bed.
It is high time that the children should go to bed.
need"不必做"和"本不該做"
didn't need to do表示： 過去不必做某事, 事實上也沒做。.
needn't have done表示： 過去不必做某事, 但事實上做了。
John went to the station with the car to meet Mary, so she didn't need to walk back home. 約翰開車去車站接瑪麗，所以她不必步行回家了。
John went to the station with the car to meet Mary, so she needn't have walked back home. 約翰開車去車站接瑪麗，所以她本不必步行回家了。 (Mary步行回家，沒有遇上John的車。)
典型例題
There was plenty of time. She ___.
A. mustn't have hurried B. couldn't have hurried
C. must not hurry D. needn't have hurried
答案D。needn't have done. 意為"本不必"，即已經做了某事，而時實際上不必要。
Mustn't have done 用法不正確，對過去發生的事情進行否定性推斷應為couldn't have
done, "不可能已經"。 must not do 不可以（用於一般現在時）
