时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
a. 同现在事实相反的假设。
句型 ： 条件从句 主句
一般过去时 should( would) +动词原形
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
如果各地方都一样，就不需要地理学家了。（Ｄ）
b. 表示于过去事实相反的假设。
句型： 条件从句 主句
过去完成时 should(would) have+ 过去分词
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.
含义：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.
含义： 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，机器的效率会大大提高。
c. 表示对将来的假想
句型： 条件从句 主句
were+ 不定式 would + 动词原形
should+ 动词原形
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，因为她想：如果她得到一份工作，她就不能经常去看她的朋友了。(B)
2.2 混合条件句
主句与从句的动作发生在不同的时间，这时主，从句谓语动词的虚拟语气形式因时间不同而不同，这叫做混合条件句。
If you had asked him yesterday, you would know what to do now.
（从句与过去事实相反，主句与现在事实相反。）
If it had rained last night (过去), it would be very cold today (现在).
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
你掉下来的时候，如果我没有站在梯子下面看着你，你现在就不会笑了。 (A)
2.3 虚拟条件句的倒装
虚拟条件句的从句部分如果含有were, should, 或had, 可将if省略，再把were, should或had 移到从句句首，实行倒装。
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：
在虚拟语气的从句中，动词'be'的过去时态一律用"were"，不用was， 即在从句中be用were代替。
If I were you, I would go to look for him. 如果我是你，就会去找他。
If he were here, everything would be all right. 如果他在这儿，一切都会好的。
_____ 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. 在虚拟条件状语中如果有were, should, had这三个词，通常将if省略，主语提前, 变成 were, should, had +主语的形式。但要注意，在虚拟条件状语从句中，省略连词的倒装形式的句首不能用动词的缩略形式。如我们可说 Were I not to do., 而不能说 Weren't I to do.
练习
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
如果我那只猫知道那条玩具鱼是用橡胶制作的，她就不会去咬它了。 (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,我本来是会帮助他的。（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
如果我们后天走之前他们到达的话，我们就举行一次丰盛的宴会。 (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,她就能在星期天到达那里。(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
如果他学习更努力些，本来是可以通过考试的。(B)
2.5 与表示命令、建议、要求的动词连用的虚拟语气
在表示命令、建议、要求的动词(如move, intend, order, request, require, suggest, propose, command , desire, prefer, insist, urge, recommend, beg, ask, advise, decide)之后的宾语从句，或名词（如suggestion, motion， proposal, order, recommendation）之后的表语从句和同位语从句，以及it is desired, it is suggested, it is ordered 等结构之后的主语从句，谓语用动词原形或“should＋动词原形”，前者在新闻文字、外交文件、提案及口语中更为常见，美国人也较爱用前一种形式。注意这种句型中的虚拟语气形式不受主句动词时态的影响。如：
Our teacher recommends that we _____ as attentive as possible when visit the museum.
A. are B. shall C. were D. be
老师劝我们在参观博物馆时尽可能地聚精会神。 (D)
We desire that the tour leader us immediately of any change in plans.
A. inform B. informs C. informed D. has informed
如果计划有变动，希望导游立即通知我们。（A）
He suggested to tomorrow's exhibition together.
A. us go B. we went C. we shall D. we go
他建议我们明天一起去参观展览。（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
我父亲没去纽约，医生建议他不要去那儿。(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
我们都赞成你的建议，把讨论推迟。(A)
It was proposed that the matter discussed at the next meeting.
A. will be B. was C. would D. be
有人建议这个问题在下一次会议上讨论。（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
有人提议：让所有的准备工作做好后该工程才动工。(C)
2.6 在important等形容词之后的that从句
在important, vital, essential, necessary, strange , incredible，advisable, preferable, urgent 等形容词之后的that从句，谓语用动词原形或“ should＋动词原形”：
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
申请表务请在截止日期前送回。（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
他必须立即还那本词典。（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
我认为把这工作分配给Tim是不明智的，因为他没有经验。(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
筹集足够的钱为该项目提供资金，这点至关重要。(A)
2.7 would rather, would prefer, would sooner, would (just) as soon 也用来表达主观愿望，它们之后的宾语从句，谓语用过去式
would rather, would sooner 也用来表达主观愿望，它们之后的宾语从句，谓语用过去式：
I'd rather you those important documents with you.
A. didn't take B. won't take
C. didn't take D. not take
我宁愿你身上不带那些重要文件。(C)
I'd sooner you didn't hurt Jim so much. He is still very depressed.
吉姆还很沮丧，你就不要那样伤他的心了。
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
坦诚地说,我宁愿你不要卷入这个案子。(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，我宁愿你先做生意。(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
我宁愿你对这个问题暂时不要发表意见。(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
坦诚地说，关于这，我宁愿你目前不要做任何事情。(A)
特别需要注意的是，如果would rather, would (just) as soon 后面不接宾语从句，而是直接和动词连用，则后面接动词原型。
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 后面的从句谓语动词要用过去式或用should加动词原形，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之后的宾语从句，if only 引起的状语从句中，谓语动词要用虚拟语气
在wish之后的宾语从句，if only 引起的状语从句中，谓语动词要用虚拟语气。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
他没有去参加聚会，但他的确希望他去了那里。（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，可惜我得起床去上课。(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！我的处境多么糟糕，要是我听了你的忠告就好了。(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
要是委员会能通过这些规定并使之尽快生效就好了。（D）
2.9 在as if/though引导的状语从句中，谓语可用虚拟语气，也可用陈述语气，要看句子的意义而定
（1）在as if引导的状语从句中，谓语可用虚拟语气，也可用陈述语气，要看句子的意义而定。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,那棵树看上去好像很久没人给它浇水了。（虚拟语气）
It looks as if it is going to rain.
好象要下雨了。（陈述语气）
（2）表示与现在事实相反或对现在事实有所怀疑，谓语用过去时；表示与过去事实相反，谓语用过去完成时。
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.