首先,要理解虛擬語氣的定義：虛擬語氣是一種不肯定的語氣或者是假設(shè)的語氣,比如if(如果,假如),as if（就像,只是像但實際上不是）.
其次,要分清楚虛擬語氣的幾種常見類型：
一.If引導(dǎo)的非真實條件句（注意if引導(dǎo)的可以實現(xiàn)的條件,稱為真實條件句）
If從句 ,主句
（將來） 1.過去式（be只能用were） ,would/should/could/might+V原
2.were to + V原 ,would/should/could/might+V原
3.should+V原 ,would/should/could/might+V原
If there should be no air,there would be no living things.
（現(xiàn)在）過去式（be只能用were） ,would/should/could/might+V原
If I were a bird,I would fly everywhere.
(過去) had done ,would/should/could/might+have done
If we had started earlier,we should not have missed the train.
(當(dāng)然,有時候是主句在前,if從句在后,規(guī)則是一樣的.)
二.從句中的虛擬語氣
1.表示愿望,建議,命令,要求等的賓語從句中,要用 “ (should) +動詞原形” ,這些詞有：
wish,suggest,order,demand,propose,command,request,recommend,require,decide,insist,desire,urge,suggestion,proposal,plan,motion,recommendation,demand,order,desire,request,requirement,insistence,advice,decision ,obligatory .
Our teacher suggested that we (should) give the old man some help.
2.在It is/was + 某些形容詞 + that 從句的句型中,that從句中的謂語動詞常用虛擬語氣,即 “should + 動詞原形”.這些形容詞有:
important,necessary,essential,imperative,natural,strange,advisable,desirable,possible,probable,astonishing,surprising,desired,suggested,requested,recommended,ordered,proposed,decided,moved等
It is important that we should get there on time.
3.在 “ It is (about/ high ) time + that (從句)” 中,謂語動詞常用過去式表表示虛擬語氣
It is high time that people learnt English.
4.在 “as if/ as though” 引導(dǎo)的狀語從句中,謂語動詞與wish的賓語從句的虛擬形式相同.
It seems as though it were already spring.
三.省略if的非真實條件句
if省去,將助動詞.情態(tài)動詞.be或have放在句首形成部分倒裝.
If I were you,I would not do those stupid things.
= Were I you,I would not do those stupid things.
If he had known about this,he would have been very sad.
= Had he known about this,he would have been very sad.
四.含蓄虛擬語氣
有時假設(shè)的情況并不以條件從句表示出來,而是暗含在上下文中,比如通過介詞短語來表示.But for（要不是） without（沒有...的話）otherwise（否則）
But for your advice,I could not have done it so successfully.
The change could not have taken place without the open-door policy.
I was so busy then,Otherwise,I would have told him the answer.
五.錯綜時間虛擬句
非真實條件句中主句和從句的謂語動詞所指時間有時可能指不同的時間,這時要根據(jù)上下文的意思采用不同的謂語動詞形式.
If I were you,I would have taken his advice.(從句指現(xiàn)在,主句指過去)
If I had taken my raincoat with me this morning,I would not be wet now.(從句指過去,主句指現(xiàn)在).
六.用在if only 引起的感嘆句中.
If only the driver didn’t drive so fast!