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）；
(2)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)
（3）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．
（4）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．
I 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。
It’s time I was going.
It’s time somebody taught you to behave yourself.
7. 由wish引起的表示愿望的虛擬語(yǔ)氣
A．用wish表示對(duì)現(xiàn)在的愿望時(shí)，它所引起的賓語(yǔ)從句中謂語(yǔ)動(dòng)詞形式為：過(guò)去式（be動(dòng)詞用were）。
B．用wish表示對(duì)將來(lái)的愿望時(shí)，它所引起的賓語(yǔ)從句中謂語(yǔ)動(dòng)詞形式為：would, could, might+ 動(dòng)詞原形。
C．wish用于對(duì)過(guò)去的事實(shí)表示一種不可能實(shí)現(xiàn)的愿望時(shí)，賓語(yǔ)從句中的謂語(yǔ)動(dòng)詞形式為：had+動(dòng)詞過(guò)去分詞或could, would + have +動(dòng)詞過(guò)去分詞。
I wish that he weren’t so lazy.
I wish I hadn't wasted so much time.
I wish she would change her mind.
8. had hoped/ planned/ thought/ wanted/ intended表示一種過(guò)去未實(shí)現(xiàn)的愿望或令人失望的事。
I had thought he had been dead for at least 20 years.
I had hoped that she would go to the U.S. and study there, but she said
she liked to stay in China.
9. would rather.
would (had) rather, would as soon, would sooner和would prefer所引起的從句中要求用過(guò)去式表示當(dāng)時(shí)或?qū)?lái)的情況，用過(guò)去完成時(shí)表示過(guò)去的情況，表示希望或婉轉(zhuǎn)的責(zé)備。例如：
I’d rather you posted the letter right away. 我倒希望你把這封信立刻寄出去。
I would prefer he didn’t stay there too long.我倒希望他不要在那兒可得太久。
I would just as soon you had returned the book yesterday.我真希望你昨天把這本書還了。
10.It’s (high, about) time that 句型中that從句中要求用虛擬語(yǔ)氣形式，即動(dòng)詞要用過(guò)去式。
It’s time that something was done about the traffic problem downtown.
It is high time that we put an end to this discussion.現(xiàn)在是我們?cè)摻Y(jié)束講座的時(shí)候了
11. 虛擬語(yǔ)氣在由某些連詞或個(gè)詞引出的表示條件、目的的狀語(yǔ)從旬中的使用。常見(jiàn)的這些連詞和介詞有：
1) unless (除非)，provided(倘若)，supposing(假若)，providing(倘若) , on condition that(如果),由這些詞、詞組引出的從句，動(dòng)詞多用過(guò)去時(shí)。
除非你命令他做，要不他是不會(huì)做的。
He wouldn't do it unless you ordered him to.
假若你有機(jī)會(huì)出國(guó)，你會(huì)去哪兒呢？
Providing you had the opportunity to go abroad, where would you go?
12. so that (以便 )，for fear that(以免, 唯恐)，in order that (以便)，whether(不管)，lest(以免)，in case (假使)，從句中加情態(tài)動(dòng)詞，有時(shí)should可省略。
他把它藏起來(lái)以免她看見(jiàn)。
He hid it lest she (should) see it.
我得準(zhǔn)備點(diǎn)啤酒，說(shuō)不定約翰會(huì)來(lái)。
I'll get some beer in case John should come.
他輕輕地進(jìn)屋，以免吵醒他的同房。
He entered the room quietly in order that he should not wake his roommate．
13. if only
如果表示對(duì)現(xiàn)在的愿望，句中的謂語(yǔ)動(dòng)詞常用過(guò)去式；
如果表示將來(lái)的愿望，句中的謂語(yǔ)動(dòng)詞常用would/could+動(dòng)詞原形；
如果表示對(duì)過(guò)去的愿望，句中的謂語(yǔ)動(dòng)詞常用had+過(guò)去分詞。
if only I were you!
If only you had come yesterday!
If only Linda could go with us tomorrow!
If only I had said more about it!
only if表示"只有"；if only則表示"如果……就好了"。If only也可用于陳述語(yǔ)氣。
I wake up only if the alarm clock rings. 只有鬧鐘響了，我才會(huì)醒。
If only the alarm clock had rung. 當(dāng)時(shí)鬧鐘響了，就好了。
If only he comes early. 但愿他早點(diǎn)回來(lái)。