4. if條件句中的虛擬語氣形式1)if非真實條件句所表示的假設則是不可能或不大可能發生或實現的，句中的主句與從句都用虛擬語氣。
在if非真實虛擬條件句中，主句和從句謂語動詞主要有下面幾種形式：
假設類型條件從句謂語動詞形式主句謂語動詞形式與現在事實相反動詞過去時(be用were)Should(would，could，might)+動詞原形與過去事實相反Had +過去分詞Should(would，could, might)+ have +過去分詞與將來事實可能相反動詞過去時(should+動詞原形，were to +動詞原形)Should(would，could，might)+動詞原形
If there were no gravity, we should not be able to walk.假如沒有引力，我們就不可能行走。
I wouldn’t have known what these were for if I hadn’t been told.假如別人不告訴我，我就不知道這些東西是干什么的了。
2)正式文體中，有時可把虛擬條件句中的連詞if省去，而將were，had, should等助動詞(不包括行為動詞)提到主語前面。如果句中沒有were，had或should時，既不能省略if，也不能倒裝。例如：
Were I to meet him tomorrow (= If I were to met him tomorrow), I should ask him about it.要是我明天見到他，我就會問他這件事的。
Had I had the money last year (= If I had had the money last year), I would have bought the house.如果我去年有了這筆錢，我就買那所房子了。
Should there be any trouble with the boiler, the automatic controlling unit would cut off the fuel oil supply.假如鍋爐出問題的話，自控裝置會自動切斷燃油的供給。
3)通常情況下，在非真實條件句中主句和從句的謂語動詞所指時間是一致的，但有時也可能指不同的時間，這時要根據上下文的意思采用不同的謂語動詞形式。例如：
If I were you, I would have taken his advice.我要是你，我就采納了他的建議。(從句指現在，主句指過去)
If the weather had been more favorable, the crop would be growing still better.如果氣候更適宜一些，莊稼會長得更好。(從句指過去，主句指現在)
5.含蓄虛擬條件句1)有時假設的情況并不以條件從句形式表示出來，而是通過一個介詞短語或其他方式表示。常用的詞或短語有：without, but for, but that, otherwise, or, but等。例如：
Without your help (=If we had not had your help), we could not have succeeded.要是沒有你的幫助，我們就不會成功的。
But for electricity (= If there were no electricity), there would be no modern industry.要是沒有電，就不會有現代工業。
He was having a meeting; otherwise he would have come over to help us.他當時正與他的學生進行討論，否則的話他就來幫我們了。
He felt very tired yesterday, or he would have attended the party.他昨天很累，不然他就參加那個聚會了。
2)在某些暗含虛擬條件的簡單句中，虛擬語氣可通過上下文表現出來。謂語動詞用should/ would be形式和should/ would have been虛擬形式。例如：
Any men in his position would have done like that.任何人處在他的位置都會那么做的。
You should (ought to) have come earlier.你本應來得早點。
6.其他句型中的虛擬語氣1)在would rather, would sooner, would just as soon, might as well和would prefer等后面所跟的從句中，也可以用虛擬語氣，表示愿望，意為“寧愿、但愿”。其形式為：
①“would rather (would sooner…) +主語+謂語動詞過去式”表示現在或將來的情況。
②“would rather (would sooner) +主語+動詞過去完成式”表示過去的情況。例如：I’d rather he didn’t go now.要是他現在不走就好了I’d just as soon you had been here yesterday.要是你昨天在這里就好了。
2)在句型“It is (high/ about) time…”后面也可以跟虛擬語氣，從句中常用過去式。有時也用“(should)+動詞原形”，意為“該是……的時候了”。例如：
It is time that the boy (should) go to school.這個小孩該上學了。
It is high time (that) the weather improved.天氣真該好起來了。
3)在“if only”引起的感嘆句中也要求用虛擬語氣，表示愿望。
形式為：①用過去時或“would/ could +動詞原形”表示與現在或未來事實相反的愿望。
②過去完成時表示與過去事實相反的愿望。例如：
If only you would listen to our advice.要是你聽我們的建議就好了。
If only I had not been busy last week!要是上周我不忙該多好啊!
If only she could marry me.但愿她能嫁給我。
注：if only也可以用于虛擬條件句中。例如：
If only I had more money, I could buy a car.要是我有更多的錢，我就可以買輛車了。
4)連接詞“in case, lest, for fear that”可以用來引導虛擬語氣。其形式通常為：“…lest (in case, for fear that) +主語+ should +動詞原形”。Lest, for fear that句中的should可以省去，in case句中的should通常不省去，但是in case句中可以不用虛擬語氣，而用陳述語氣。Lest, for fear that后面也可以接其他形式。例如：
He took his coat with him in case it should rain.他帶著雨衣以防下雨。
I will not make a noise for fear that I (should/ might) disturb you.我不會做聲的，以免打擾你。
Care must be taken in using this method lest overflow (should) occur.在使用這以方法的時候要謹慎小心，以免發生溢流現象。
