1．虛擬語(yǔ)氣在條件句中的用法
（1）若表示與現(xiàn)在事實(shí)相反的假設(shè),則條件從句中用過(guò)去式,主句中用過(guò)去將來(lái)式（would,should,could,might + 動(dòng)詞原形）；若表示某事將來(lái)實(shí)現(xiàn)的可能性不大,則條件從句中用should + 動(dòng)詞原形也可用“were to+動(dòng)詞原形”或用過(guò)去式動(dòng)詞.如：
I would certainly go if I had time．（現(xiàn)在）
If it should rain,we wouldn”t go out.（將來(lái)）
If I knew German,I would read the paper to you．（現(xiàn)在）
If he were here,we could ask him．（現(xiàn)在）
（2）若表示與過(guò)去事實(shí)相反的假設(shè),從句中用過(guò)去完成式,主句中用過(guò)去將來(lái)完成式（should,would,could,might + have + 過(guò)去分詞）.如：
If I had set off a little earlier,I would have caught the train．
She would / might have come if she hadn”t been so busy．
If I hadn”t taken your advice,I would have made a bad mistake．
If only you hadn”t told him the news．
（3）含有虛擬語(yǔ)氣的條件狀語(yǔ)從句中,如有had,should,were 這三個(gè)詞的話,在正式或書(shū)面語(yǔ)言中可將if省略,再將句子的主語(yǔ)和謂語(yǔ)動(dòng)詞實(shí)行全部倒裝或部分倒裝.如：
Had they time（=If they had 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 I were you）,I would go．
（4）若主句從句所指的時(shí)間不一致,即條件從句表示與過(guò)去事實(shí)相反,主句表示與現(xiàn)在事實(shí)相反；或者條件從句表示與現(xiàn)在事實(shí)相反,主句表真實(shí)情況,則從句中應(yīng)采用與具體時(shí)間相對(duì)應(yīng)的虛擬形式.如：
If it had rained last night（過(guò)去）,It would be very cold today（現(xiàn)在）．
If I had taken your advice（過(guò)去）,I should have no trouble with the work now（現(xiàn)在）．
2．虛擬語(yǔ)氣在賓語(yǔ)從句中的用法
　　在動(dòng)詞suggest,order,demand,propose,request,command,insist等后的賓語(yǔ)從句中,用虛擬語(yǔ)氣（即Should+動(dòng)詞原形或只用動(dòng)詞原形）來(lái)表示愿望、建議、命令、請(qǐng)求等.在動(dòng)詞wish /would rather后的賓語(yǔ)從句中,用過(guò)去式表示與現(xiàn)在事實(shí)不符,用過(guò)去完成式表示與過(guò)去事實(shí)不符.如：
I suggest that we（should）set off at once．
The doctor insisted that the patient (should)be X-rayed．
I wish I were as strong as you．
would rather that you hadn”t told him．
3．虛擬語(yǔ)氣在主語(yǔ)從句中的用法
　　　在It is necessary/important/strange/natural；It is requested/suggested/desired/proposed；it is a pity等結(jié)構(gòu)后的主語(yǔ)從句中要用虛擬語(yǔ)氣,即Should+動(dòng)詞原形或只用動(dòng)詞原形.如：
It is necessary that he（should）be sent there at once．
It is requested that Professor Li（should）give us a speech．
It is desired that we（should）get everything ready by tonight．
4．虛擬語(yǔ)氣在表語(yǔ)從句、同位語(yǔ)從句中的用法.
　　在 suggestion,proposal,idea,plan,order,advice等名詞后的表語(yǔ)從句、同位語(yǔ)從句中要用虛擬語(yǔ)氣,即should+動(dòng)詞原形或只用動(dòng)詞原形.如：
Our suggestion is that you（ should）be the first to go．
My advice is that we（should）send for Doctor Li．
Do you know the order that you（should）keep watch?
5．虛擬語(yǔ)氣在其他狀語(yǔ)從句中的用法
　　在as if引導(dǎo)的狀語(yǔ)從句中,用過(guò)去式表示與現(xiàn)在事實(shí)不符,用過(guò)去完成式表示與過(guò)去事實(shí)不符；在so that,in order that引導(dǎo)的狀語(yǔ)從句中,常用should/would/could/might+動(dòng)詞原形來(lái)表示虛擬語(yǔ)氣.如：
They talked as if they had been friends for years．
She stayed at home for a few days so that she could take care of her mother．
He took a taxi to the station so that she should not miss the train．
6．虛擬語(yǔ)氣在定語(yǔ)從句中的用法：
在it is time后面的定語(yǔ)從句中常用過(guò)去式表示虛擬