1.虛擬語氣在條件句中的用法
(1)若表示與現在事實相反的假設，則條件從句中用過去式，主句中用過去將來式（would，should，could，might + 動詞原形）；
(2)若表示某事將來實現的可能性不大，則條件從句中用should + 動詞原形也可用“were to+動詞原形”或用過去式動詞。如：
I would certainly go if I had time．（現在）
If it should rain，we wouldn't go out.（將來
（3）若表示與過去事實相反的假設，從句中用過去完成式，主句中用過去將來完成式（should，would，could，might + have + 過去分詞）。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）含有虛擬語氣的條件狀語從句中，如有had，should，were 這三個詞的話，在正式或書面語言中可將if省略，再將句子的主語和謂語動詞實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間不一致，即條件從句表示與過去事實相反，主句表示與現在事實相反；或者條件從句表示與現在事實相反，主句表真實情況，則從句中應采用與具體時間相對應的虛擬形式。如：
If it had rained last night（過去），It would be very cold today（現在）．
If I had taken your advice（過去），I should have no trouble with the work now（現在）．
2．虛擬語氣在賓語從句中的用法
在動詞suggest，order，demand，propose，request，command，insist等后的賓語從句中，用虛擬語氣（即Should+動詞原形或只用動詞原形）來表示愿望、建議、命令、請求等。在動詞wish /would rather后的賓語從句中，用過去式表示與現在事實不符，用過去完成式表示與過去事實不符。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．虛擬語氣在主語從句中的用法
在It is necessary/important/strange/natural；It is requested/suggested/desired/proposed；it is a pity等結構后的主語從句中要用虛擬語氣，即Should+動詞原形或只用動詞原形。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．虛擬語氣在表語從句、同位語從句中的用法。
在 suggestion， proposal，idea，plan，order，advice等名詞后的表語從句、同位語從句中要用虛擬語氣，即should+動詞原形或只用動詞原形。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．虛擬語氣在其他狀語從句中的用法
在as if引導的狀語從句中，用過去式表示與現在事實不符，用過去完成式表示與過去事實不符；
在so that，in order that引導的狀語從句中，常用should/would/could/might+動詞原形來表示虛擬語氣。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．虛擬語氣在定語從句中的用法：
在it is time后面的定語從句中常用過去式表示虛擬。
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氣
A．用wish表示對現在的愿望時，它所引起的賓語從句中謂語動詞形式為：過去式（be動詞用were）。
B．用wish表示對將來的愿望時，它所引起的賓語從句中謂語動詞形式為：would, could, might+ 動詞原形。
C．wish用于對過去的事實表示一種不可能實現的愿望時，賓語從句中的謂語動詞形式為：had+動詞過去分詞或could, would + have +動詞過去分詞。
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表示一種過去未實現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去式表示當時或將來的情況，用過去完成時表示過去的情況，表示希望或婉轉的責備。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氣形式，即動詞要用過去式。
It’s time that something was done about the traffic problem downtown.
It is high time that we put an end to this discussion.現在是我們該結束講座的時候了
11. 虛擬語氣在由某些連詞或個詞引出的表示條件、目的的狀語從旬中的使用。常見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，動詞多用過去時。
除非你命令他做，要不他是不會做的。
He wouldn't do it unless you ordered him to.
假若你有機會出國，你會去哪兒呢？
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)，從句中加情態動詞，有時should可省略。
他把它藏起來以免她看見。
He hid it lest she (should) see it.
我得準備點啤酒，說不定約翰會來。
I'll get some beer in case John should come.
他輕輕地進屋，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
如果表示對現在的愿望，句中的謂語動詞常用過去式；
如果表示將來的愿望，句中的謂語動詞常用would/could+動詞原形；
如果表示對過去的愿望，句中的謂語動詞常用had+過去分詞。
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也可用于陳述語氣。
I wake up only if the alarm clock rings. 只有鬧鐘響了，我才會醒。
If only the alarm clock had rung. 當時鬧鐘響了，就好了。
If only he comes early. 但愿他早點回來。