英語虛擬語氣一直受到語言學家及語法學家們的關注。他們從各個方面對虛擬語氣進行了分析，包括給虛擬語氣下定義，進行分類，討論其在交際中的功能，甚至也爭論其存在的價值，而隨着語言學發展，虛擬語氣的意義構建成為研究重點。學好虛擬語氣對於學習英語的整體有着極大的幫助!
(1)若表示與現在事實相反的假設，則條件從句中用過去式，主句中用過去將來式(would，should，could，might + 動詞原形);若表示某事將來實現的可能性不大，則條件從句中用should + 動詞原形也可用“were to+動詞原形”或用過去式動詞。
I would certainly go if I had time.(現在)
If it should rain，we wouldn’t go out.(將來)
If I knew German，I would read the paper to you.(現在)
If he were here，we could ask him.(現在)
(2)若表示與過去事實相反的假設，從句中用過去完成式，主句中用過去將來完成式(should，would，could，might + have + 過去分詞)。
If Ihad set off a little earlier，I would have caught the train.
Shewould / might have come if she hadn’t been so busy.
If I hadn’t taken your advice，I wouldhave made a bad mistake.
If only you hadn’t told him the news.
(3)含有虛擬語氣的條件狀語從句中，如有had，should，were這三個詞的話，在正式或書面語言中可將if省略，再將句子的主語和謂語動詞實行全部倒裝或部分倒裝。如：
Had they time(=If theyhad time)，they would certainly come and help us.
Should it be fine(=If it should be fine)，we would go for an outing.
Were I go(=If Iwere you)，I would go.
(4)若主句從句所指的時間不一致，即條件從句表示與過去事實相反，主句表示與現在事實相反;或者條件從句表示與現在事實相反，主句表示真實情況，則從句中應採用與具體時間相對應的虛擬形式。
If ithad rained last night(過去)，It would be very cold today(現在).
If Ihad taken your advice(過去)，I should have no trouble with the work now(現在).
(1) 將來時的條件句中的虛擬語氣。
If he should go to Qing Hua University, he would make full use of his time. 如果他要上清華大學的話，他就會充分利用他的時間了。
If he were to come here, he would tell us about it. 如果他要來的話，他會通知我們一聲。
(2) 現在時的條件句中的虛擬語氣。
If he were free, he would help us. 要是他有空的話，它會幫助我們的。
If he studied at this school, he would know you well. 如果他在這所學校學習的話，它會對你很熟悉。
(3) 過去時的條件句中的虛擬語氣。
If I had seen the film, I would have told you about it. 我如果看過這場電影，我會把電影內容告訴你了。
If I had got there earlier, I would have met Mr. Li. 如果我早點到那兒，我就會會到了李先生。
(1) 當從句的主語為第三人稱單數時，謂語動詞若是系動詞be時，可用was 代替were。但在倒裝虛擬結構及if I were you, as it were中，只能用were。
Were I ten years younger, I would study abroad. 要是我還年輕十歲的話，我會去國外學習。
If I were you, I would try my best to grasp the chance. 要是我是你的話，我要盡力抓住這次機會。
(2) 有時，虛擬條件句中，主、從句的動作若不是同時發生時，須區別對待。
①從句的動作與過去事實相反，而主句的動作與現在或現在正在發生的事實不符。
If I had worked hard at school, I would be an engineer, too. 如果我在學校學習刻苦的話，我現在也會使工程師了
If they had informed us, we would not come here now. 如果他們通知過我們的話，我們現在就不會來這裏了。
②從句的動作與現在事實相反，而主句的動作與過去事實不符。
If he were free today, we would have sent him to Beijing. 如果他今天有空的話，我們會已經派他去北京了。
If he knew her, he would have greeted her. 要是他認識她的話，他肯定會去問候她了。
③從句的動作與過去發生的情況相反，而主句的動作與現在正在發生的情況相反。
If it had not been raining too much, the crops would be growing much better. 如果天不下太多的雨的話，莊家會長得更好。
If he had been working hard, he would be working in the office now. 要是他工作一直努力的話，他現在已進了辦公室了。
一、表示與現在事實相反的情況：
從句：If 主語+過去時(Be動詞用were)
主句：主語+should/would/could/might+do
eg：
I were you,I would take an umbrella.
如果我是你，我會帶把傘。(事實：我不可能是你)
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：不知道)
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：地球上既有空氣也有水)
I had any money with me,I could lend you some.
如果我帶錢了，我就會借給你些。(事實：沒有帶錢)
he studied harder,he might pass the exam.
如果他再努力些，就能通過考試了。(事實：沒有努力)
(注意：一般在虛擬的條件句中無論是第幾人稱，用到系動詞的過去式時，只用were而不用was)
二、表示與過去事實相反的情況
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：去晚了)
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：沒有聽我的話)
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：我並非陌生人)
三、表示對將來情況的主觀推測
主句：主語+would+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：
he should come here tomorrow,I would talk to him.
如果他明天來這兒的話，我就跟他談談。(事實：來的可能性很小)
there were a heavy snow next Sunday,we would not go skating.
如果下週日下大雪，我們就不能去滑冰了。(事實：下雪可能性很小)
she were to be here next Monday,I would tell her about the matter.
如果她下週一來這兒的話，我就會告訴她這件事的始末。(事實：來的可能性很小)
四、有時，虛擬條件句中，如果主句和條件從句的謂語動作若不是同時發生時，虛擬語氣的形式應作出相應的調整。這種條件句叫錯綜條件句。
①從句的動作與過去事實相反，而主句的動作與現在或現在正在發生的事實不符。
eg：
If I had worked hard at school,I would be an engineer,too.
如果我在學校學習刻苦的話，我現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，我們現在就不會來這裏了。
②從句的動作與現在事實相反，而主句的動作與過去事實不符。
如：
If he were free today,we would have sent him to Beijing.
如果他今天有空的話，我們會已經派他去北京了。
If he knew her,he would have greeted her.
要是他認識她的話，他肯定會去問候她了。
五、當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等詞置於句首，這種多用於書面語。eg：
Should he agree to go there,we would send him there.
要是他答應去的話，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，我們已經聘用他來這裏工作了。
六、非真實條件句中的條件從句有時不表達出來，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，我不會取得如此大的進步。
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。
七、有時，虛擬條件句中，主、從句可以省略其中的一個，來表示説話人的一種強烈的感情。
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. 要是我現在在家裏該多好啊。
If only I had got it. 要是隻有我得到它了該多好啊。
八.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。
eg：If I were you,I would go to look for him.
如果我是你，就會去找他。
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