時態：可以表示過去，現在和將來的情況。它的基本特點是時態退后。
a. 同現在事實相反的假設。
句型 ： 條件從句 主句
一般過去時 should( would) +動詞原形
If they were here, they would help you.
If places _____ alike, there would be little need for geographers.
A. being B. are C. be D. were
如果各地方都一樣，就不需要地理學家了。（Ｄ）
b. 表示于過去事實相反的假設。
句型： 條件從句 主句
過去完成時 should(would) have+ 過去分詞
If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
If he had come yesterday, I should / would have told him about it.
含義：He did not come yesterday, so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress.
含義： He was ill and missed many lessons, so he did not make greater progress.
If the policeman had arrived earlier, he would have seen the accident.
If better material had been used, the efficiency of the machine would have been much increased.
如果使用更好的材料，機器的效率會大大提高。
c. 表示對將來的假想
句型： 條件從句 主句
were+ 不定式 would + 動詞原形
should+ 動詞原形
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
If it should rain tomorrow, we should have to put off the picnic.
Jean doesn't want to work right away because she thinks that if she ____ a job, she probably wouldn't be able to see her friends very often.
A) has to get B) were to get
C) had got D) could have got
吉不想立刻工作，因為她想：如果她得到一份工作，她就不能經常去看她的朋友了。(B)
2.2 混合條件句
主句與從句的動作發生在不同的時間，這時主，從句謂語動詞的虛擬語氣形式因時間不同而不同，這叫做混合條件句。
If you had asked him yesterday, you would know what to do now.
（從句與過去事實相反，主句與現在事實相反。）
If it had rained last night (過去), it would be very cold today (現在).
If I hadn't stood under the ladder to catch you when you fell , you ____ now.
A) wouldn't be smiling B) couldn't have smiled
C) won't smile D) didn't smile
你掉下來的時候，如果我沒有站在梯子下面看著你，你現在就不會笑了。 (A)
2.3 虛擬條件句的倒裝
虛擬條件句的從句部分如果含有were, should, 或had, 可將if省略，再把were, should或had 移到從句句首，實行倒裝。
Were they here now, they could help us. =If they were here now, they could help us.
Had you come earlier, you would have met him =If you had come earlier, you would have met him.
Should it rain, the crops would be saved. =Were it to rain, the crops would be saved.
注意：
在虛擬語氣的從句中，動詞'be'的過去時態一律用"were"，不用was， 即在從句中be用were代替。
If I were you, I would go to look for him. 如果我是你，就會去找他。
If he were here, everything would be all right. 如果他在這兒，一切都會好的。
_____ to do the work, I should do it some other day.
A. If were I B. I were C. Were I D. Was I
答案C. 在虛擬條件狀語中如果有were, should, had這三個詞，通常將if省略，主語提前, 變成 were, should, had +主語的形式。但要注意，在虛擬條件狀語從句中，省略連詞的倒裝形式的句首不能用動詞的縮略形式。如我們可說 Were I not to do., 而不能說 Weren't I to do.
練習
My cat wouldn't have bitten the toy fish _____ it was made of rubber.
A) if she has known B) if she knew
C) she should have know D) had she known
如果我那只貓知道那條玩具魚是用橡膠制作的，她就不會去咬它了。 (D）
____ for my illness I would have lent him a helping hand.
A) Not being B) Had it not been
C. Without being D) Not having been
如果不是我病了,我本來是會幫助他的。（B)
____ before we depart the day after tomorrow, we should have a wonderful dinner party.
A) Had they arrived B) Would they arrive
C) Were they arriving D) Were they to arrive
如果我們后天走之前他們到達的話，我們就舉行一次豐盛的宴會。 (D）
____ right now, she would get there on Sunday.
A) Would she leave B) If she leave
C) Were she to leave C) If she had left
如果她立刻就走,她就能在星期天到達那里。(C)
Had he worked harder, he ____ the exams.
A) must have got through B) would have got through
C) would get though D) could get through
如果他學習更努力些，本來是可以通過考試的。(B)
2.5 與表示命令、建議、要求的動詞連用的虛擬語氣
在表示命令、建議、要求的動詞(如move, intend, order, request, require, suggest, propose, command , desire, prefer, insist, urge, recommend, beg, ask, advise, decide)之后的賓語從句，或名詞（如suggestion, motion， proposal, order, recommendation）之后的表語從句和同位語從句，以及it is desired, it is suggested, it is ordered 等結構之后的主語從句，謂語用動詞原形或“should＋動詞原形”，前者在新聞文字、外交文件、提案及口語中更為常見，美國人也較愛用前一種形式。注意這種句型中的虛擬語氣形式不受主句動詞時態的影響。如：
Our teacher recommends that we _____ as attentive as possible when visit the museum.
A. are B. shall C. were D. be
老師勸我們在參觀博物館時盡可能地聚精會神。 (D)
We desire that the tour leader us immediately of any change in plans.
A. inform B. informs C. informed D. has informed
如果計劃有變動，希望導游立即通知我們。（A）
He suggested to tomorrow's exhibition together.
A. us go B. we went C. we shall D. we go
他建議我們明天一起去參觀展覽。（Ｄ）
My father didn't go to New York, the doctor suggested that he ____ there.
A. not to go B. not go
B. C. hadn't gone D. wouldn’t go
我父親沒去紐約，醫生建議他不要去那兒。(B)
We are all for your proposal that the discussion ____.
A) be put off B) was put off
C) should put off D) is to put off
我們都贊成你的建議，把討論推遲。(A)
It was proposed that the matter discussed at the next meeting.
A. will be B. was C. would D. be
有人建議這個問題在下一次會議上討論。（Ｄ）
It is recommended that the project ____ until all the preparations have been made.
A) is not started B) will not be started
C) not be started D) is not to be started
有人提議：讓所有的準備工作做好后該工程才動工。(C)
2.6 在important等形容詞之后的that從句
在important, vital, essential, necessary, strange , incredible，advisable, preferable, urgent 等形容詞之后的that從句，謂語用動詞原形或“ should＋動詞原形”：
It was essential that the application forms back before the deadline.
A. must be sent B. would be sent
C. be sent D. were sent
申請表務請在截止日期前送回。（Ｃ)
It is necessary the dictionary immediately.
A. that he will return B. that he returned
C. that he return D. that he had to return
他必須立即還那本詞典。（C）
I don't think it advisable that Tim ___ to the job since he has no experience.
A) in assigned B) will be assigned
C) be assigned D) has been assigned
我認為把這工作分配給Tim是不明智的，因為他沒有經驗。(C)
It is vital that enough money ____ to fund the subject.
A) be collected B) must be collected
C) is collected D) can be collected
籌集足夠的錢為該項目提供資金，這點至關重要。(A)
2.7 would rather, would prefer, would sooner, would (just) as soon 也用來表達主觀愿望，它們之后的賓語從句，謂語用過去式
would rather, would sooner 也用來表達主觀愿望，它們之后的賓語從句，謂語用過去式：
I'd rather you those important documents with you.
A. didn't take B. won't take
C. didn't take D. not take
我寧愿你身上不帶那些重要文件。(C)
I'd sooner you didn't hurt Jim so much. He is still very depressed.
吉姆還很沮喪，你就不要那樣傷他的心了。
To be frank, I'd rather you ____ in the case.
A) will not be involved B. not involved
c) not to be involved D. were not involved
坦誠地說,我寧愿你不要卷入這個案子。(D)
You don't have to be in such a hurry, I would rather you ___on business first.
A) would go B) will go
C) went D) have gone
你不必如此匆忙，我寧愿你先做生意。(C)
I'd rather you ___ make any comment on the issue for the time being。
A) don't B) wouldn't C) didn't D) shouldn't
我寧愿你對這個問題暫時不要發表意見。(C)
Frankly speaking, I'd rather you ____ anything about it for the time being.
A) didn't do B) haven't done
C) don't do D) have done
坦誠地說，關于這，我寧愿你目前不要做任何事情。(A)
特別需要注意的是，如果would rather, would (just) as soon 后面不接賓語從句，而是直接和動詞連用，則后面接動詞原型。
I would rather go there tomorrow.
He would rather not stay at home tonight.
另，It is (high) time that It is (high) time that 后面的從句謂語動詞要用過去式或用should加動詞原形，但should不可省略。
It is time that the children went to bed.
It is high time that the children should go to bed.
2.8 在wish之后的賓語從句，if only 引起的狀語從句中，謂語動詞要用虛擬語氣
在wish之后的賓語從句，if only 引起的狀語從句中，謂語動詞要用虛擬語氣。如：
He didn't go to the party, but he does wish he _____there.
A) had been B) has been
C) would have been D) would be
他沒有去參加聚會，但他的確希望他去了那里。（Ａ)
I wish I ___ longer this morning, but I had to get up and come to class.
A) could have slept B) slept
C) might have slept D) have slept
今天早上我能睡得更久些就好了，可惜我得起床去上課。(A)
Look at the situation I'm in! If only I ___ your advice.
A) follow B) had followed
C) would follow D) have followed
瞧！我的處境多么糟糕，要是我聽了你的忠告就好了。(B)
If only the committee ____ the regulations and put them into effect as soon as possible.
A) approve B) will approve
C) can approve D) would approve
要是委員會能通過這些規定并使之盡快生效就好了。（D）
2.9 在as if/though引導的狀語從句中，謂語可用虛擬語氣，也可用陳述語氣，要看句子的意義而定
（1）在as if引導的狀語從句中，謂語可用虛擬語氣，也可用陳述語氣，要看句子的意義而定。如：
That tree looked as if it _____ for a long time.
A) hasn't watered B) didn't water
C) hadn't been watered D) wasn't watered
（Ｃ,那棵樹看上去好像很久沒人給它澆水了。（虛擬語氣）
It looks as if it is going to rain.
好象要下雨了。（陳述語氣）
（2）表示與現在事實相反或對現在事實有所懷疑，謂語用過去時；表示與過去事實相反，謂語用過去完成時。
He talks to me as if he were my brother.
He told us his early experience in New York as if he had really been a millionaire there.
They talked (are talking) as if they had been friends for years.
It seems as if it was (were) spring today.
He acts (acted) as if (though) he were (was) an expert.