虚拟语气用来表示说话人的主观愿望或假想，而不表示客观存在的事实，所说的是一个条件，不一定是事实，或与事实相反。虚拟语气通过谓语动词的特殊形式来表示。英语中的语气分为陈述语气、祈使语气、虚拟语气三类。
在非真实条件状语从句中的用法
真实条件状语从句与非真实条件状语从句
条件句可分为两类，一类为真实条件句，一类为非真实条件句。非真实条件句表示的是假设或实际可能性不大的情况，故采用虚拟语气。
eg：
If he doesn't hurry up,he will miss the bus.如果他不快点，他将错过巴士。( 真实)
If he is free, he will ask me to tell stories.如果他是空闲的，他会要求我讲故事。（真实）
If I were you, I would go at once.如果我是你，我马上就会去。 （我不是你。非真实，虚拟语气）
If there were no air, people would die.如果没有空气，人就会死亡。（不可能没有空气。非真实，虚拟语气）
用法及动词形式
1、表示与现在事实相反的情况：
从句：主语+过去时
主句：主语+should/would/could/might+do
eg：
1.If I were you, I would take an umbrella.
如果我是你，我会带把伞。（事实：我不可能是你）
2.If I knew his telephone number, I would tell you.
如果我知道他的电话号码，我就会告诉你。（事实：不知道）
3.If there were no air or water, there would be no living things on the earth.
如果没有水和空气，地球上就不会有生物。(事实：地球上既有空气也有水)
4.If I had any money with me, I could lend you some.
如果我带钱了，我就会借给你些。（事实：没带钱）
5.If he studied harder, he might pass the exam.
如果他再努力些，就能通过考试了。（事实：没有努力）
2、表示与过去事实相反的情况
从句：主语+had done
主句：主语+should/would/could/might+have done
eg：
1. If I had gotten there earlier, I should/could have met her. 如果我早到那儿，我就会见到她。
（事实：去晚了）
2.If he had taken my advice, he would not have made such a mistake. 如果他听我的劝告的话，就不会犯这样的错误了。
（事实：没有听我的话）
3、表示对将来情况的主观推测
从句：①if+主语+were to do 主句：①主语+should/would/could/might+do
②if+主语+did/were ②主语+should/would/could/might+do
③if+主语+should+do ③主语+should/would/could/might+do
eg：
1.If he should come here tomorrow, I would talk to him.
如果他明天来这儿的话，我就跟他谈谈。 （事实：来的可能性很小）
2.If there were a heavy snow next Sunday, we would not go skating.
如果下周日下大雪，我们就不能去滑冰了。（事实：不知能否下雪）
3.If she were to be here next Monday, I would tell her about the matter.
如果她下周一来这儿的话，我就会告诉她这件事的始末。
4、 有时，虚拟条件句中，结果主句和条件从句的谓语动作若不是同时发生时，虚拟语气的形式应作相应的调整。
①从句的动作与过去事实相反，而主句的动作与现在或现在正在发生的事实不符。
eg：
If I had worked hard at school, I would be an engineer, too.
如果我在学校学习刻苦的话，我现在也会成为一个工程师了
If they had informed us, we would not come here now.
如果他们通知过我们的话，我们现在就不会来这里了。
②从句的动作与现在事实相反，而主句的动作与过去事实不符。
如：
If he were free today, we would have sent him to Beijing.
如果他今天有空的话，我们会已经派他去北京了。
If he knew her, he would have greeted her.
要是他认识她的话，他肯定会去问候她了。
5、 当虚拟条件句的谓语动词含有were, should, had时，if可以省略，这时条件从句要用倒装语序，即把were, should, had等词置于句首，这种多用于书面语。 eg：
Should he agree to go there, we would send him there.
要是他答应去的话，我们就派他去。
Were she here, she would agree with us.
如果她在这儿的话，她会同意我们的。
Had he learnt about computers, we would have hired him to work here.
如果他懂一些电脑知识的话，我们已经聘用他来这里工作了。
6、非真实条件句中的条件从句有时不表达出来，只暗含在副词、介词短语、上下文或其他方式表示出来，这种句子叫做含蓄条件句，在多数情况下，条件会暗含在短语中，如without…, but for…等
eg：
But for his help, we would be working now.
要不是他的帮助，我们还会在工作呢。
Without your instruction, I would not have made such great progress.
要是没有你的指导，我不会取得如此大的进步。
We didn't know his telephone number, otherwise we would have telephoned him.
我们不知道他的电话号码，否则我们就会给他打电话。
7、 有时，虚拟条件句中，主、从句可以省略其中的一个，来表示说话人的一种强烈的感情。
①省略从句
He would have finished it. 他本该完成了。
You could have passed this exam. 你本该通过这次考试的。
②省略主句
If I were at home now. 要是我现在在家里该多好啊。
If only I had got it. 要是只有我得到它了该多好啊。
8.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。
eg：If I were you,I would go to look for him.
如果我是你，就会去找他。
编辑本段
虚拟语气（Subjunctive Mood)的其他用法
用在wish 后的宾语从句
a、表示与现在事实相反的愿望，谓语动词用过去式
eg. I wish I had your brains.
我希望我有你那样的头脑。(事实：我根本比不上你)
b、表示与过去事实相反的愿望，谓语动词：had+done
eg:.I wish I had known the truth of the matter.
我希望我那时就知道这件事情的真相。（事实：那时还不知道）
c、表示将来难以实现的愿望
谓语动词：should/would + 动词原形
eg. I wish I should have a chance again.
我希望我还能有一次这样的机会。（事实：很难再有这样的机会了）
(注：if only和as if/as though也有相同用法）
用在目的状语从句中
1、在for fear that, in case, lest引导的从句中，若用虚拟语气时，从句谓语为：should + do。并且 should不能省略
She examined the door again for fear that a thief should come in.
她又把门检查了一遍，以防盗贼的进入。
He started out earlier lest he should be late.
他早早的就出发了以防迟到。
2、在so that, in order that所引导的目的状语从句中，从句中的谓语为：can / could / may / might / will / would / should + do。
He goes closer to the speaker so that he can hear him clearer. 他走近说话的人以便能听得更清楚。
He read the letter carefully in order that he should not miss a word. 他把信读得很仔细以便不漏掉一个字。
其他用法
1、一想要(desire)二宁愿(prefer)三命令(order. command)四建议(advice. suggest. propose)五要求(demand. require. request. ask.insist)中，无论主句谓语动词为何种时态，从句的谓语动词都用："should + do"。should可以省略。eg：
He suggested that we (should) takethe teacher's advice.
He insisted that we (should) takethe teacher's advice.
He demand that we (should) takethe teacher's advice.
He ordered that we (should) takethe teacher's advice.
insist意为“坚持某种动作”才用虚拟语气；意为“坚持某种观点，某个事实”则不用虚拟语气。
eg：
He insists he is a student.
他坚持说他是个学生。
这个语句表示的是事实，因此在这个语句中不能使用虚拟语气。
suggest意为“建议”才用虚拟语气，意为“暗示”则不用虚拟语气。
eg：
His face suggests that he looks worried .
他的表情暗含着他很担心。
这个句子本身是事实，因此它就没有用到虚拟语气。
2、表情绪.观点的形容词或名词也要用虚拟语气.如:necessary、important、impossible、natural、strange、surprising、funny、right、wrong、better、a pity等。
句型：It is.......that +主语从句，从句的谓语动词都要用should+原型 或只用动词原型。
eg:
Do you think it necessary that he (should) not be sent to Lingbao.
It is srange that such a person should be our friend.
奇怪的是这样一个人会成为我们的朋友。
注：这一点还没有准确的说法，希望善心人能把这点补充完全。
3、 在even if, even though 所引导的让步状语从句中，可用虚拟语气，主句、从句的结构与if所引导的条件从句结构相同。
eg：
Even if he were here himself, he should not know what to do.
即使他亲自来也不知该怎么办。（事实：他没来）
Nobody could save him even though Hua Tuo should come here.
即使华佗在世也救不了他。（事实：华佗不在世）
4、 在whatever，whichever，whenever，whoever, wherever，however, no matter wh-等引导的让步状语从句中，从句虚拟语气结构为：
指现在或将来： may +do。
eg：
We will finish it on time no matter what may happen.
不管发生什么事，我们都要按时完成。
We will find him wherever he may be.
无论他在哪里，我们都要找到他。
I will wait for him no matter how late he may come.
不管他来的多么晚，我都会等他。
指过去： may +have done 。
eg：
You mustn't be proud whatever great progress you may have made.
不管你取得了多么大的进步，你也不能骄傲。
We must respect him no matter what mistakes he may have made.
不管他犯过什么错误，我们必须尊敬他。
5、一般would rather, had rather, would sooner等之后的宾语从句常表示与客观事实不相符的一种愿望，故使用虚拟语气。其虚拟语气的结构为：
过去 had + done
现在 过去时(be 用were )
将来 过去时(be 用were )
eg：
I'd rather you had seen the film yesterday.
我倒想你昨天看过了这场电影。
I'd rather you were here now.
我倒想你现在在这儿。
We'd rather you went here tomorrow.
我们倒想你明天去那儿
注：注意would rather，had rather，would sooner的细微差别，可以百度一下查查。
6、虚拟语气还可用在定语从句中，表示：“早该做某事了”时，定语从句中的谓语动词须用虚拟语气，其虚拟语气的结构为：It is (high / about) time that + 主语+ 动词的过去式/ should + do，即从句用虚拟过去式。
It is time that I went to pick up my daughter at school.
我该去学校接我的女儿了。
It is high time you should go to work.
你早该上班了。
7、 简单句中的虚拟语气
（1） 说话时，为了表示客气、谦虚、委婉而有礼貌，言语常使用虚拟语气。其虚拟语气的结构形式常为：would / could / might / should +do。
eg：
Would you mind my shutting the door?
你介意我把门关起来吗？
You should always learn this lesson by heart.
你要把这个教训牢记于心。
I should agree with you.
我本该同意你的观点。（委婉的不同意）
（2） 表示“祝愿”时，常用may + 主语+ do。
eg：
May you have a good journey!
祝你一路顺风。
May your youth last for ever!
祝你青春永驻。
（3）表示强烈的“愿望”、“祝愿”时，常用do。
eg：
Long live the Communist Party of China.
中国共产党万岁。
God bless us.
上帝保佑。
（4） 习惯表达中常用的虚拟语气。
① 提出请求或邀请。eg：
Would you like to have a talk with us this evening?
今天晚上来跟我们聊天好吗？
Could I use your bike now?
我可以用一下你的单车吗？
② 陈述自己的观点或看法。eg：
I should glad to meet you.
见到你我会很高兴。
I would try my best to help you.
我会尽力帮助你。
③提出劝告或建议。eg：
You'd better ask your father first.
你最好先问一问你的父亲。
You should make a full investigation of it first.
你应该先全面调查一番。
④ 提出问题。eg：
Do you think he could get here on time?
你认为他能按时来吗？
Do you expect he would tell us the truth?
你期望他会告诉我们真相吗？
⑤ 表示对过去情况的责备时，常用虚拟语气。其虚拟语气的结构为：情态动词 + have done。
eg：
You should have got here earlier. 你早该到这里了。
You should have returned it to him. 你早该把它还给他了。
8、虚拟语气在方式状语从句的应用。详见百度百科之方式状语从句词条。
编辑本段
四、省略的虚拟条件句型
省略连词if
有时可将条件从句的连词if省略，但此时应用倒装句型，即把从句中的were, should, had 等提到句首：
Were I Tom, I would refuse. 如果我是汤姆，我会拒绝。
Should it be necessary,I would go. 假若有必要，我会去的。
Had it not been for the bad weather we would have arrived on time. 若不是天气坏，我们就准时到达了。
【注】
① 若条件从句为否定句，否定词not应置于主语之后，而不能与were, should, had 等缩略成Weren’t, Shouldn’t, Hadn’t而置于句首。
② 有时省略if后提前的had不是助动词：
Had I time, I would come. 假若我有时间，我会来的。(=If I had time…)
省略条件句的主语和其后的动词be
若主从句主语一致，且谓语部分包含有动词be，通常可将主语和动词be省略：
If repaired earlier, the tractor would not have broken down.
要是早点儿修一下，拖拉机就不会抛锚了。(=If it had been repaired earlier…)
省略“it+be”
If necessary, I would send more farmhands to help you.
如果需要的话，我会派更多的人去帮你。(=If it was necessary,…)
省略整个条件从句
这样的省略通常需要借助一定的上下文，即省略条件从句后，所剩下的主句的意思在一定的上下文中意思是清楚的：
I might see her personally. It would be better. 我可以亲自去看她，这样好一些。(=If I saw her personally, it would be better.)
编辑本段
在含蓄条件句中的用法
(1) 条件暗含在短语中。如：
We didn’t know his telephone number; otherwise we would have telephoned him.
我们不知道他的电话号码，否则我们就会给他打电话。（暗含条件是otherwise）
Without your help, we wouldn’t have achieved so much.
没有你的帮助，我们不可能取得这么大的成绩。
（暗含条件是介词短语without your help）
But for your help, I would not have succeeded in the experiment.
如果没有你的帮助，我的实验就不会成功。（暗含条件是but for your help）
It would cause great trouble not to lubricate the bearing immediately.
不立即润滑轴承就会引起很大的故障。
（暗含条件是not to lubricate the bearing immediately）
(2) 条件暗含在上下文中。如：
I would not have done it that way.
我是不会那么做的。（可能暗含if I were you）
I was busy that day. Otherwise I would have come to help you.
我那天很忙。否则我会来帮你的。（可能暗含if I hadn’t been so busy.）
You might come to join us in the discussion.
你可以参加我们的讨论。（可能暗含if you wanted to）
I would have bought the DVD player.
我是会买下那台影碟机的。（可能暗含if I had the money）
But for the storm, we should have arrived earlier.
要不是碰到暴雨，我们还会早些到。（可能暗含if it had not been for the storm）。
虚拟语气是高二语法的一个重点，用法如下：
虚拟条件句的三种基本类型：与现在事实相反、与过去事实相反、与将来事实相反。
条件句有真实条件句和非真实(虚拟)条件句两种。真实条件句所表示的假设是有可能发生的，而非真实条件句则通常表示一种假想，与事实相反或不大可能会发生：
If I have time, I will go with them. 假若我有时间，我就同他们去。(陈述语气)
If I were you, I would go with them. 假若我是你，我就同他们去。(虚拟语气)
▲ 与现在事实相反
若与现在事实相反，条件从句的谓语用过去式(be通常用were)，主句谓语用“should (would, could, might)+动词原形”：
If I knew her number ，I could ring her up. 要是我知道她的电话号码，我就可以给她打电话了。(可惜我不知道)
▲与过去事实相反
若与过去事实相反，条件从句的谓语用过去完成时(had+过去分词)，主句谓语用“should (would, could, might)+have+过去分词”：
If I’d left sooner，I’d have been on time. 要是我早点动身，我就准时到了。(但我动身太迟了)
▲与将来事实相反
若与将来事实相反，条件从句的谓语用过去式(be通常用were)，主句谓语用“should (would, could, might)+动词原形”：
If I asked him，I’m sure he’d help us. 如果我向他提出要求，肯定他会帮助我们。(不过我不打算这样做)
听老师之间的对话学习虚拟语气
虚拟语气用来表示说话人的主观愿望或假想，而不表示客观存在的事实，所说的是一个条件，不一定是事实，或与事实相反。虚拟语气通过谓语动词的特殊形式来表示。英语中的语气分为陈述语气、祈使语气、虚拟语气三类。
http://wenku.baidu.com/view/f54057c58bd63186bcebbc83.html
这个归纳的挺全的 还可以练习 平常多练多读 再看看语法书 就行了
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