虛擬語氣
1.主語從句中的虛擬語氣主語從句中的虛擬語氣主要取決于某些形容詞和過去分詞,用來表示建議、命令、要求、驚異和失望等.其形式為：（should）+動(dòng)詞原形.
常這樣用的形容詞有：
appropriate適當(dāng)?shù)腶dvisable適當(dāng)?shù)?合理的better較好的desirable理想的essential精華的imperative迫切的important重要的insistent堅(jiān)持的natural自然的necessary必要的preferable優(yōu)越的,較好的strange奇怪的urgent緊迫的vital極其重要的
過去分詞有：
desired, demanded, ordered, requested, suggested, recommended, required等.
例如：
It is essential that you (should) win the voters’hearts.贏得選民的心是絕對(duì)必要的.
It was suggested that he (should) join the club activities.有人提議他應(yīng)該參加俱樂部的活動(dòng).
2.表語從句和同位語從句中的虛擬語氣在表示建議、命令、主張、目的和愿望等名詞后面的表語從句和同位語從句中謂語動(dòng)詞要求使用虛擬語氣.其形式為：（should）+動(dòng)詞原形.
常這樣用的名詞有：
resolution決心,決議pray懇求decision決議motion提議suggestion建議preference選擇proposal提議advice勸告recommendation推薦desire愿望demand要求requirement要求order命令necessity必要性request要求idea主意、想法例如：
He gave orders that the guests (should) be hospitably entertained.他下令熱情款待這些客人.
His proposal is that we (should) get rid of the bad habits.他提議我們應(yīng)該擯棄這些不良習(xí)性.
3.賓語從句中的虛擬語氣
1）某些動(dòng)詞后所接賓語從句中的謂語動(dòng)詞要求用虛擬語氣,用來表示建議、命令和要求等.其形式為：（should）+動(dòng)詞原形.常這樣用的動(dòng)詞有：
ask要求advise勸告determine決定decide決定command命令insist堅(jiān)持intend打算move提議prefer寧愿propose提議order命令recommend推薦request要求require要求suggest建議urge主張demand要求desire渴望direct命令例如：
He insisted that the meeting be put off.他要求推遲那個(gè)會(huì)議.
They proposed that all the plans should be discussed at the meeting.他們建議所有的計(jì)劃都應(yīng)該在會(huì)上討論.
2）在以it為形式賓語的復(fù)合賓語從句中也可以用虛擬語氣,其形式為：（should）+動(dòng)詞原形.這一句型中使用的形容詞（賓補(bǔ)）與主語從句中的虛擬語氣所使用的形容詞相同.例如：
We think it advisable that he (should) think deeply before acting.我們認(rèn)為他在行動(dòng)之前好好考慮一下才是明智的.
3）wish后面所跟的賓語從句中的虛擬語氣表示某種不能實(shí)現(xiàn)的愿望.
其主要形式有三種：
表示對(duì)現(xiàn)在情況的虛擬：wish +主語+動(dòng)詞過去式或were表示對(duì)過去情況的虛擬：wish +主語+ had +過去分詞表示對(duì)將來情況的虛擬：wish +主語+ would +動(dòng)詞原形.
例如：
I wish I knew what was going to happen.但愿我能知道要發(fā)生什么事.
She wished she had stayed at home.她懊悔的是她當(dāng)時(shí)要是在家就好了.
I wish I were rich.我恨不得我很有錢.
注意：在表示對(duì)過去情況的虛擬時(shí),有時(shí)也可以用下列形式：
wish +主語+ would（could）+ have +過去分詞I wish I could have seen her last night.要是我昨天晚上能夠見到她那該多好.
4. if條件句中的虛擬語氣形式1）if非真實(shí)條件句所表示的假設(shè)則是不可能或不大可能發(fā)生或?qū)崿F(xiàn)的,句中的主句與從句都用虛擬語氣.
在if非真實(shí)虛擬條件句中,主句和從句謂語動(dòng)詞主要有下面幾種形式：
假設(shè)類型條件從句謂語動(dòng)詞形式主句謂語動(dòng)詞形式與現(xiàn)在事實(shí)相反動(dòng)詞過去時(shí)（be用were）Should（would,could,might）+動(dòng)詞原形與過去事實(shí)相反Had +過去分詞Should（would,could, might）+ have +過去分詞與將來事實(shí)可能相反動(dòng)詞過去時(shí)（should+動(dòng)詞原形,were to +動(dòng)詞原形）Should（would,could,might）+動(dòng)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(shí)可把虛擬條件句中的連詞if省去,而將were,had, should等助動(dòng)詞（不包括行為動(dòng)詞）提到主語前面.如果句中沒有were,had或should時(shí),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(huì)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(huì)自動(dòng)切斷燃油的供給.
3）通常情況下,在非真實(shí)條件句中主句和從句的謂語動(dòng)詞所指時(shí)間是一致的,但有時(shí)也可能指不同的時(shí)間,這時(shí)要根據(jù)上下文的意思采用不同的謂語動(dòng)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(xiàn)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(huì)長得更好.（從句指過去,主句指現(xiàn)在）
5.含蓄虛擬條件句1）有時(shí)假設(shè)的情況并不以條件從句形式表示出來,而是通過一個(gè)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(huì)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(huì)有現(xiàn)代工業(yè).
He was having a meeting; otherwise he would have come over to help us.他當(dāng)時(shí)正與他的學(xué)生進(jìn)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(gè)聚會(huì)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(xiàn)出來.謂語動(dòng)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(huì)那么做的.
You should (ought to) have come earlier.你本應(yīng)來得早點(diǎn).
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(dòng)詞過去式”表示現(xiàn)在或?qū)淼那闆r.
②“would rather (would sooner) +主語+動(dòng)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(xiàn)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(shí)也用“（should）+動(dòng)詞原形”,意為“該是……的時(shí)候了”.例如：
It is time that the boy (should) go to school.這個(gè)小孩該上學(xué)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(shí)或“would/ could +動(dòng)詞原形”表示與現(xiàn)在或未來事實(shí)相反的愿望.
②過去完成時(shí)表示與過去事實(shí)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(dǎo)虛擬語氣.其形式通常為：“…lest (in case, for fear that) +主語+ should +動(dòng)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(huì)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(shí)候要謹(jǐn)慎小心,以免發(fā)生溢流現(xiàn)象.