用來表示說話人的主觀愿望或假想，而不表示客觀存在的事實(shí)，所說的是一個條件，不一定是事實(shí)，或與事實(shí)相反。
虛擬語氣通過謂語動詞的特殊形式來表示。
初中階段主要學(xué)習(xí)在條件句中的虛擬語氣。
條件句中的虛擬語氣根據(jù)不同的時間有三種不同的形式。
時間 | 條件從句 | 主句 | 例句 |
與現(xiàn)在事實(shí)相反 | If+主語+過去式（系 | would | 見①②句 |
與過去事實(shí)相反 | If+主語+had+過去分詞 | Would | 見③④句 |
與將來事實(shí)相反 | 過去式 | would | 見⑤⑥句 |
例句：
① If I were rich, I would travel around the world.
② If I won a million dollars, I would give it to charities.
③ If you had taken my advice, you wouldn’t have failed.
④ If I had known that earlier, I wouldn’t have done it.
⑤ If he should go to Tsinghua University, he would make full use of his time.
⑥ If he were to come here, he would tell us about it.
虛擬語氣誤區(qū):
1.混合條件句的主從句時態(tài)不會靈活變化；
2.省略if時，句子調(diào)整不正確；
3.不會去找意思中的“應(yīng)該”含義；
4.陷在虛擬中出不來，把真實(shí)條件句當(dāng)成虛擬條件句。
省略虛擬條件:
1.省略連詞if
有時可將條件從句的連詞if省略，但此時應(yīng)用倒裝句型，即把從句中的were,should,had 等提到句首：
Were I Tom,I would refuse. 如果我是湯姆，我會拒絕。
Should it be necessary,I would go. 假若有必要，我會去的。
Had it not been for the bad weather we would have arrived on time. 若不是天氣壞，我們就準(zhǔn)時到達(dá)了。
【注】
① 若條件從句為否定句，否定詞not應(yīng)置于主語之后，而不能與were,should,had 等縮略成Weren’t,Shouldn’t,Hadn’t而置于句首。
② 有時省略if后提前的had不是助動詞：
Had I time,I would come. 假若我有時間，我會來的。(=If I had time…)
2.省略主語和動詞be
若主從句主語一致，且謂語部分包含有動詞be，通�？蓪⒅髡Z和動詞be省略：
If repaired earlier,the tractor would not have broken down.
要是早點(diǎn)兒修一下，拖拉機(jī)就不會拋錨了。(=If it had been repaired earlier…)
3.省略“it+be”
If necessary,I would send more farmhands to help you.
如果需要的話，我會派更多的人去幫你。(=If it was necessary,…)
4.省略條件從句
這樣的省略通常需要借助一定的上下文，即省略條件從句后，所剩下的主句的意思在一定的上下文中意思是清楚的：
I might see her personally. It would be better.
我可以親自去看她，這樣好一些。(=If I saw her personally,it would be better.)
含蓄條件句中
(1) 條件暗含在短語中。如：
We didn’t know his telephone number; otherwise we would have telephoned him.
我們不知道他的電話號碼，否則我們就會給他打電話。（暗含條件是otherwise）
Without your help,we wouldn’t have achieved so much.
沒有你的幫助，我們不可能取得這么大的成績。
（暗含條件是介詞短語without your help）
But for your help,I would not have succeeded in the experiment.
如果沒有你的幫助，我的實(shí)驗(yàn)就不會成功。（暗含條件是but for your help）
It would cause great trouble not to lubricate the bearing immediately.
不立即潤滑軸承就會引起很大的故障。
（暗含條件是not to lubricate the bearing immediately）
(2) 條件暗含在上下文中。如：
I would not have done it that way.
我是不會那么做的。（可能暗含if I were you）
I was busy that day. Otherwise I would have come to help you.
我那天很忙。否則我會來幫你的。（可能暗含if I hadn’t been so busy.）
You might come to join us in the discussion.
你可以參加我們的討論。（可能暗含if you wanted to）
I would have bought the DVD player.
我是會買下那臺影碟機(jī)的。（可能暗含if I had the money）
But for the storm,we should have arrived earlier.
要不是碰到暴雨，我們還會早些到。（可能暗含if it had not been for the storm）。
條件狀語從句用法及動詞形式:
1、表示與現(xiàn)在事實(shí)相反的情況：
從句：If 主語+過去時(Be動詞用were)
主句：主語+should/would/could/might+do
eg：
1．If I were you,I would take an umbrella.
如果我是你，我會帶把傘。（事實(shí)：我不可能是你）
2．If I knew his telephone number,I would tell you.
如果我知道他的電話號碼，我就會告訴你。（事實(shí)：不知道）
3．If there were no air or water,there would be no living things on the earth.
如果沒有水和空氣，地球上就不會有生物。(事實(shí)：地球上既有空氣也有水)
4．If I had any money with me,I could lend you some.
如果我?guī)уX了，我就會借給你些。（事實(shí)：沒有帶錢）
5．If he studied harder,he might pass the exam.
如果他再努力些，就能通過考試了。（事實(shí)：沒有努力）
6．she looked at me as if I had been a stranger.
她看我的樣子好像我是一個陌生人。（事實(shí)：我并非陌生人）
2、表示與過去事實(shí)相反的情況
從句：If 主語+had+done
主句：主語+should/would/could/might+have done
eg：
1．If I had got there earlier,I should/could have met her. 如果我早到那兒，我就會見到她。（事實(shí)：去晚了）
2．If he had taken my advice,he would not have made such a mistake. 如果他聽我的勸告的話，就不會犯這樣的錯誤了。（事實(shí)：沒有聽我的話）
3、表示對將來情況的主觀推測
主句：主語+should+do
從句：
①if+主語+were to do
②if+主語+should/would/could/might+do
③if+主語+did（動詞過去式）/were
eg：
1．If he should come here tomorrow,I would talk to him.
如果他明天來這兒的話，我就跟他談?wù)�。（事�?shí)：來的可能性很小）
2．If there were a heavy snow next Sunday,we would not go skating.
如果下周日下大雪，我們就不能去滑冰了。（事實(shí)：下雪可能性很�。�
3．If she were to be here next Monday,I would tell her about the matter.
如果她下周一來這兒的話，我就會告訴她這件事的始末。（事實(shí)：來的可能性很�。�
4、 有時，虛擬條件句中，結(jié)果主句和條件從句的謂語動作若不是同時發(fā)生時，虛擬語氣的形式應(yīng)作相應(yīng)的調(diào)整。這種條件句叫錯綜條件句。
①從句的動作與過去事實(shí)相反，而主句的動作與現(xiàn)在或現(xiàn)在正在發(fā)生的事實(shí)不符。
eg：
If I had worked hard at school,I would be an engineer,too.
如果我在學(xué)校學(xué)習(xí)刻苦的話，我現(xiàn)在也會成為一個工程師了
If they had informed us,we would not come here now.
如果他們通知過我們的話，我們現(xiàn)在就不會來這里了。
②從句的動作與現(xiàn)在事實(shí)相反，而主句的動作與過去事實(shí)不符。如：
If he were free today,we would have sent him to Beijing.
如果他今天有空的話，我們會已經(jīng)派他去北京了。
If he knew her,he would have greeted her.
要是他認(rèn)識她的話，他肯定會去問候她了。
5、 當(dāng)虛擬條件句的謂語動詞含有were,should,had時，if可以省略，這時條件從句要用倒裝語序，即把were,should,had等詞置于句首，這種多用于書面語。eg：
Should he agree to go there,we would send him there.
要是他答應(yīng)去的話，我們就派他去。
Were she here,she would agree with us.
如果她在這兒的話，她會同意我們的。
Had he learnt about computers,we would have hired him to work here.
如果他懂一些電腦知識的話，我們已經(jīng)聘用他來這里工作了。
6、非真實(shí)條件句中的條件從句有時不表達(dá)出來，只暗含在副詞、介詞短語、上下文或其他方式表示出來，這種句子叫做含蓄條件句，在多數(shù)情況下，條件會暗含在短語中，如without…，but for…等
eg：
But for his help,we would be working now.
要不是他的幫助，我們還會在工作呢。
Without your instruction,I would not have made such great progress.
要是沒有你的指導(dǎo)，我不會取得如此大的進(jìn)步。
We didn't know his telephone number,otherwise we would have telephoned him.
我們不知道他的電話號碼，否則我們就會給他打電話。
7、有時，虛擬條件句中，主、從句可以省略其中的一個，來表示說話人的一種強(qiáng)烈的感情。
①省略從句
He would have finished it. 他本該完成了。
You could have passed this exam. 你本能通過這次考試的。
②省略主句
If I were at home now. 要是我現(xiàn)在在家里該多好啊。
If only I had got it. 要是只有我得到它了該多好啊。
8.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。
eg：If I were you,I would go to look for him.
如果我是你，就會去找他。
虛擬語氣的其他用法:
1、一想要( desire )
一寧愿( prefer )
一堅(jiān)持( insist )
二命令( order , command )
三建議( advise , suggest , propose/recommend)
四要求( demand , require , request , ask)中，
無論主句謂語動詞為何種時態(tài)，從句的謂語動詞都用："should + do"。should可以省略。
eg：
He suggested that we (should) take the teacher's advice.
He insisted that we (should) take the teacher's advice.
He demand that we (should) take the teacher's advice.
He ordered that we (should) take the teacher's advice.
insist 意為“堅(jiān)持某種動作”才用虛擬語氣；意為“堅(jiān)持某種觀點(diǎn)，某個事實(shí)”則不用虛擬語氣。eg：
He insists he is a student.
他堅(jiān)持說他是個學(xué)生。
這個語句表示的是事實(shí)，因此在這個語句中不能使用虛擬語氣。
suggest意為“建議”才用虛擬語氣，意為“暗示”則不用虛擬語氣。
eg：
His face suggests that he looks worried .
他的表情暗含著他很擔(dān)心。
這個句子本身是事實(shí)，因此它就沒有用到虛擬語氣。
2、表情緒、觀點(diǎn)的形容詞或名詞也要用虛擬語氣。如：
necessary、important、impossible、natural、strange、surprising、funny、right、wrong、better、a pity ,the shame ,no wonder等。
句型：It is.......that +主語從句，從句的謂語動詞都要用 should+原型或 只用動詞原型。
eg:
Do you think it is necessary that he (should)not be sent to Lingbao.
It is strange that such a person should be our friend.
奇怪的是這樣一個人會成為我們的朋友。
注：這一點(diǎn)還沒有準(zhǔn)確的說法，希望善心人能把這點(diǎn)補(bǔ)充完全。
3、 在even if,even though 所引導(dǎo)的讓步狀語從句中用may/might+動詞原形，may/might可以省略，表示與現(xiàn)在相反的情況；
從句用過去完成時，表示與過去相反的情況，類似的詞有though/even though/whatever/however/so long as; 主句、從句的結(jié)構(gòu)與if所引導(dǎo)的條件從句結(jié)構(gòu)相同。
eg：
Even if he were here himself,he should not know what to do.
即使他親自來也不知該怎么辦。（事實(shí)：他沒來）
Nobody could save him even though Hua Tuo should come here.
即使華佗在世也救不了他。（事實(shí)：華佗不在世）
4、 在whatever，whichever，whenever，whoever,wherever，however,no matter wh-等引導(dǎo)的讓步狀語從句中，從句虛擬語氣結(jié)構(gòu)為：指現(xiàn)在或?qū)恚簃ay +do。
eg：
We will finish it on time no matter what may happen.
不管發(fā)生什么事，我們都要按時完成。
We will find him wherever he may be.
無論他在哪里，我們都要找到他。
I will wait for him no matter how late he may come.
不管他來的多么晚，我都會等他。
指過去：may +have done 。
eg：
You mustn't be proud whatever great progress you may have made.
不管你取得了多么大的進(jìn)步，你也不能驕傲。
We must respect him no matter what mistakes he may have made.
不管他犯過什么錯誤，我們必須尊敬他。
5、一般would rather,had rather,would sooner等之后的賓語從句常表示與客觀事實(shí)不相符的一種愿望，故使用虛擬語氣。其虛擬語氣的結(jié)構(gòu)為：
過去 had + done
現(xiàn)在 過去時(be 用were )
將來 過去時(be 用were ) （would rather 將來情況用一般過去時）
eg：
I'd rather you had seen the film yesterday.
我倒想你昨天看過了這場電影。
I'd rather you were here now.
我倒想你現(xiàn)在在這兒。
We'd rather you went here tomorrow.
我們倒想你明天去那兒
注：注意would rather，had rather，would sooner的細(xì)微差別，可以百度一下查查。
6、虛擬語氣還可用在定語從句中，表示：“早該做某事了”時，定語從句中的謂語動詞須用虛擬語氣，其虛擬語氣的結(jié)構(gòu)為：
It is (high / about) time that + 主語+ 動詞的過去式/ should + do（優(yōu)先使用動詞過去式） ，即從句用虛擬過去式。
It is time that I went to pick up my daughter at school.
我該去學(xué)校接我的女兒了。
It is high time you should go to work.
你早該上班了。
7、 簡單句中的虛擬語氣
（1） 說話時，為了表示客氣、謙虛、委婉而有禮貌，言語常使用虛擬語氣。其虛擬語氣的結(jié)構(gòu)形式常為：would / could / might / should +do。
eg：
Would you mind me shutting the door?
你介意我把門關(guān)起來嗎？
You should always learn this lesson by heart.
你要把這個教訓(xùn)牢記于心。
I should agree with you.
我本該同意你的觀點(diǎn)。（委婉的不同意）
（2）表示“祝愿”時，常用may + 主語+ do。
eg：
May you have a good journey!
祝你一路順風(fēng)。
May your youth last forever!
祝你青春永駐。
（3）表示強(qiáng)烈的“愿望”、“祝愿”時，常用do。
eg：God bless us. 上帝保佑。
（4）習(xí)慣表達(dá)中常用的虛擬語氣。
① 提出請求或邀請。eg：
Would you like to have a talk with us this evening?
今天晚上來跟我們聊天好嗎？
Could I use your bike now?
我可以用一下你的單車嗎？
② 陳述自己的觀點(diǎn)或看法。eg：
I should be glad to meet you.
見到你我會很高興。
I would try my best to help you.
我會盡力幫助你。
③提出勸告或建議。eg：
You'd better ask your father first.
你最好先問一問你的父親。
You should make a full investigation of it first.
你應(yīng)該先全面調(diào)查一番。
④ 提出問題。eg：
Do you think he could get here on time
你認(rèn)為他能按時來嗎？
Do you expect he would tell us the truth?
你期望他會告訴我們真相嗎？
⑤表示對過去情況的責(zé)備時，常用虛擬語氣。其虛擬語氣的結(jié)構(gòu)為：情態(tài)動詞 + have done。
eg：
You should have got here earlier. 你早該到這里了。
You should have returned it to him. 你早該把它還給他了。
8、虛擬語氣在方式狀語從句的應(yīng)用。詳見百度百科之方式狀語從句詞條。
本文來自：逍遙右腦記憶 http://m.douzifanli.com/chuzhong/274921.html
相關(guān)閱讀：初中英語知識點(diǎn)：被動語態(tài)