1）虛擬語氣用來表示說話人的主觀愿望或假想，所說的是一個條件，不一定是事實，或與事實相反。虛擬語氣在條件句中應用比較多。
2）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?qū)嶋H可能性不大的情況，故采用虛擬語氣。
1 真實條件句用于陳述語氣，假設的情況有可能發(fā)生。各種結(jié)構(gòu)參見下表：
句型 條件從句 主句
一般現(xiàn)在時 shall/will + 動詞原形
祈使句
情態(tài)動詞一般現(xiàn)在時
例如：If he comes, he will bring his violin. 如果他來，會帶小提琴來的。
典型例題:
The volleyball match will be put off if it ___.
A. will rain B. rains C. rained D. is rained
答案B。真實條件句主句為將來時，從句用一般現(xiàn)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，主句謂語動詞不用shall （will） +動詞原形，而直接用一般現(xiàn)在時的動詞形式。
2 非真實條件句
1）虛擬語氣可以表示過去，現(xiàn)在和將來的情況，時態(tài)的基本特點是時態(tài)往后推移。
a. 與現(xiàn)在事實相反的假設
條件從句 主句
一般過去時（be用were） should（would）等 +動詞原形
例如：If they were here, they would help you. 如果他們在這兒，會幫助你的。
含義：They are not here, they can’t help you.
b. 與過去事實相反的假設
條件從句 主句
過去完成時 should（would）等+ have+ 過去分詞
例如：If he had come yesterday, I should / would have told him about it.如果他昨天來的話，我會把這件事告訴他的。
含義：He did not come yesterday, so I did not tell him about it.
c. 表示對將來不大可能發(fā)生的事情的假想
條件從句 主句
一般過去時 should/would等 + 動詞原形
were+ 不定式
should+ 動詞原形
例如：If you succeeded, everything would be all right. 如果你將來成功了，一切都會好的。
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
含義：You are not likely to succeed, everything will be what it is now.
3 混合條件句
有時，主句與從句的動作發(fā)生在不同的時間，主句從句謂語動詞的虛擬語氣形式因時間不同而不同，這種條件句叫做混合條件句。例如：
If you had asked him yesterday, you would know what to do now. 如果你昨天問過他，今天就知道做什么了。
（從句與過去事實相反，主句與現(xiàn)在事實相反。）
If it had rained last night（過去）, it would be very cold today （現(xiàn)在）.如果昨晚下過雨，今天就會很冷了。
4 虛擬條件句的倒裝
虛擬條件句的從句部分含有were, should, 或had時, 可省略if，再把were, should或had 移到從句的句首，實行倒裝。例如：
Were they here now, they could help us. =If they were here now, they could help us.他們現(xiàn)在在的話，就會幫助我們了。
Had you come earlier, you would have met him. =If you had come earlier, you would have met him.你來得早一點，就碰到他了。
Should it rain, the crops would be saved. =Were it to rain, the crops would be saved.假如下雨，莊稼就有救了。
注意：在虛擬語氣的從句中，動詞'be'的過去時態(tài)一律用"were"，不用was，即在從句中be用were代替。例如：
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 were C. Were I D. 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。
5 特殊的虛擬語氣詞should
1）在主語從句中的應用
It is demanded / necessary / a pity + that…等結(jié)構(gòu)的主語從句，謂語動詞用should 加動詞原形，should 可省略。
It is 可用的詞有三類 that （should）do
suggested, ordered, required, proposed, demanded, requested, insisted等
important, necessary, natural, imperative, strange等
a pity, a shame, no wonder等
2）在賓語從句中的應用
在表示命令、建議、要求等一類動詞后面的從句中，像order, suggest, propose, require, demand, request, insist, command, insist + （should） do等。例如：
I suggest that we （should） hold a meeting next week. 我建議下周召開個會議。
He insisted that he （should ） be sent there.他要求被派到那兒去。
注意：如suggest, insist不表示"建議" 或"堅持要某人做某事時"，即它們用于其本意"暗示、表明"、"堅持認為"時，賓語從句用陳述語氣。
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
suggestion, proposal, idea, plan, order, advice等名詞后面的表語從句、同位語從句中要用虛擬語氣，即（should）+動詞原形。例如：
My idea is that we（should）get more people to attend the conference.我的想法是讓更多的人來參加會議。
I make a proposal that we（should）hold a meeting next week.我提了個建議，下周我們開個會。
6 wish的用法
1）wish后面的從句，表示與事實相反的情況，或表示將來不太可能實現(xiàn)的愿望。其賓語從句的動詞形式為：
主句 從句
從句動作先于主句動詞動作（be的過去式為 were） 現(xiàn)在時 過去時
從句動作與主句動作同時發(fā)生（had + 過去分詞） 過去時 過去完成時
將來不大可能實現(xiàn)的愿望 將來時 would/could +動詞原形
例如：I wish I were as tall as you. 我希望和你一樣高。
He wished he hadn't said that. 他希望他沒講過那樣的話。
I wish it would rain tomorrow. 我希望明天下雨就好了。
2）wish to do；wish sb / sth to do。例如：
I wish to see the manager. = I want to see the manager. 我希望見一見經(jīng)理。
I wish the manager to be informed at once. （= I want the manager to be informed at once.）我希望經(jīng)理能馬上得到消息。
7 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也可用于陳述語氣。例如：
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. 當時鬧鐘響就好了。
If only he comes early. 但愿他早點回來。
8 It is （high） time that
It is （high） time that 后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。例如：
It is time that the children went to bed. 孩子們該睡覺了。
It is high time that the children should go to bed.
9 need "不必做"和"本不必做"
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。例如：
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步行回家了，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(jīng)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(fā)生的事情進行否定性推斷應為couldn't have done, "不可能已經(jīng)"。 must not do 不可以（用于一般現(xiàn)在時）。