虚拟语气是一种动词形式，表示说话人的一种愿望，假设，怀疑，猜测， 建议等含义，虚拟语气所表示的含义不是客观存在的事实。在条件句中的应用，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的或实际可能性不大的情况，故采用虚拟语气。
一、虚拟语气在从句中的使用
虚拟语气即表达不可能发生或与预期相反的情况。首先我们应熟悉它在三种时态里的使用情况，这是正确使用虚拟语气的基础。这三种时态是：现在；过去；将来。
1、表示现在的虚拟式
时 态从 句 动 词主 句 动 词
现 在 did would + do
If I were a bird, I would fly to you. 如果我是只鸟，我就会飞到你身边去。
If it were not for their help, we should be in a very difficult position.
如果不是他们的帮忙，我们就会处于一个非常困难的境地。
2、表示过去的虚拟式
时 态从 句 动 词主 句 动 词
过 去had + donewould + have + done
If we had started earlier, we should not have missed the train.
如果我们早一点动身的话，就不会误车了。
If she hadn’t been ill, she might have come.如果她不是病了，可能会来的
3、表示将来的虚拟式
时 态从 句 动 词主 句 动 词
将 来should + do
were to + dowould + do
If Professor Li should have time tomorrow, we could ask him some questions.
如果李教授明天有空的话，我们可以问他点问题。
If there should be no air, there would be no living things.
如果没有空气，就不会有生物。
二、错综时间的虚拟句
通常情况下,在非真实条件句中主句和从句的谓语动词所指时间是一致的, 但有时也可能指不同的时间,这时要根据上下文的意思采用不同的谓语动词形式.
If I were you, I would have taken his advice. (从句指现在, 主句指过去)。
换成我是你，我就会听从他的建议。
If I had taken my raincoat with me this morning, I would not be wet now. (从句指过去,主句指现在)。如果我今天早上带了雨衣，现在就不会被淋湿了。
If we hadn’t been working hard in the past few years, things wouldn’t be going so smoothly.
(从句指过去，主句指现在)。如果过去几年里不努力工作，现在的情况就不会如此顺利了。
If the weather had been more favorable, the crops would be growing still better.(从句指过去，主句指现在)。如果当时的天气更好点，现在的庄稼就会长得更好。
If we had not got everything ready by now, we should be having a more terrible time tomorrow. (从句指现在，主句指将来)
如果现在一切还没准备好，明天情况就更糟了。
三、含蓄条件句
有时假设的情况并不以条件从句表示出来,二是暗含在上下文中,比如通过介词短语来表示。这些介词常是：but for, without, otherwis等。如：
But for your advice, I could not have done it so successfully.
如果不是听了你的建议，我就不会做的如此成功。
The change could not have taken place without the open-door policy.
没有开放的政策，就不会有如此大的变化。
I was so busy then, Otherwise, I would have told him the answer.
我当时很忙，否则，我就会把答案告诉他的。
四、虚拟语气在某些从句中的应用
1、wish后的宾语从句(would rather (sooner) 后也是如此)
I would rather they came tomorrow. 我宁愿他们明天来。
How I wish it wasn’t raining now. 如果现在不下雨该多好呀！
I wish I had not been to yesterday’s party. 我要不去参加昨天的宴会就好了！
但would rather 后没有主语则应使用零不定式.
2、suggest, order, demand, propose, command, request, desire, insist 等动词后的宾语从句
They demanded that the aggressor troops (should) be withdrawn immediately.
他们要求侵略者立刻撤退。
I propose that we (should) set a deadline for handing in the plan.
我提议应该制定一个交计划的最后期限。
3、it is desired, it’s suggested, it’s requested, it was ordered that, it was proposed that, it is necessary that, it is important that, it has been decided that 等之后的主语从句：
It is necessary that he (should) be sent there at once. 有必要立刻派他前往。
It’s required that Mr. Lee (should) give a performance at the party.
要求李先生在晚会上出个节目。
It’s desired that we (should) get everything ready by tonight.
要求我们在今晚前把一切准备好。
4、在suggestion, motion, proposal, order, recommendations, plan, idea 等后面引出的表语从句和同位语从句中：
The order came that all villagers evacuate from the village.
要求所有村民撤退的命令下达了。
I second Xiao Li’s motion that we set up a special board to tackle the problem.
小李提议成立一个特别委员会处理此问题，我附议。
5、在下列句型中要用 should +动词原形。如：it is a pity, it is a shame, it is incredible, it is strange, it is no wonder
It is a great pity that he should be so conceited.
很可惜，他这个人竟然这么自大。
It’s strange that she should have failed to see her own shortcomings.
奇怪的是他竟看不到自己的缺点。
6、as if, as though 引起的从句，其用法同 wish 构成的宾语从句一样
I remember the whole thing as if it happened yesterday.
我清楚地记得这件事就象昨天刚刚发生的。
It seems as if it were spring already 似乎现在已经是春天的样子了。
7、以 lest, for fear that, in case 引出的从句，动词多用 should + 动词原形，表示 “惟恐”的意思。
He took an umbrella with him lest it should rain. 他带上雨伞以免下雨。
He burned all the important documents for fear that they should fall into the enemy's hands.
他把所有的重要文件全部烧毁以免落入敌人的手中。
Here’s some money in case you should need it. 这是点钱以免你用地着。
8、在 if only 引出的感叹句中
If only I hadn’t lost it! 如果我要不丢该多好！
If only I could see him once! 再能看到他一次该有多好！
Oh, if he only knew how we miss him! 噢，希望他能知道我是多么思念他！
9、在 it is high time that 之后的定语从句中，谓语动词常用过去式表表示虚拟语气。
It is time that we went (should go) to bed. 现在该上床了。
It’s high time that we (should) put an end to this controversy.
现在该是结束这场争辩的时候了。
10、用于expect, believe, think, suspect 等动词的否定或疑问形式后的宾语从句中。
I never expected that the new apartment should be so small. 没料到这桩新公寓竟这么小。
五、虚拟语气强化练习
1. I’d just as soon ____ rudely to her.
A. you not speak B. your not speaking
C. you didn’t speak D. you hadn’t spoken
2. _________, I’d have done it myself.
A. If I would have known it B. If I had have known it
C. Had I known it D. Should I known it
3. We prefer that the plan ____________ before being put into execution.
A.will be fully discussed B. be fully discussed
C. must be fully discussed D. would be fully discussed
4. If only we ____ then the disease was curable.�
A. knew B. could know � C. had known D. would have known
5.If you _________ my advice, you ________ your failure now. You _________ your victory.�
A. took ... wouldn’t cry over ... would celebrate�
B. had taken ... wouldn’t have cried over ... would have celebrated�
C. had taken ... aren’t crying over ... are celebrating�
D. had taken ... wouldn’t be crying over ... would be celebrating
6.It was urgent that he _________ her immediately.
A.calls B.called C.call D.would call
7. It’s high time that something _________ to prohibit selling fake commodities.
A. must be done B.was done C. be done D. were done
8. He insisted that we all ____ in his office at one o’clock.
� A. be B. to be C. would be D. shall be
9. ____________, I’d have done it myself.
A. If I would have known it B. If I had have known it
C. Had I known it D. Should I known it
10. There was a traffic jam; otherwise I ____ here on time.�
A. would be B. had been �
C. should be D. would have been
六、虚拟语气练习详解
1.【答案】C。
【解析】 would just as soon (=would rather, had rather或had just as soon) 意为 “宁愿” 后接从句时，其从句谓语动词要用一般过去时表示虚拟语气。
2.【答案】C 。
【解析】 if 条件句中包含有 should, had,were 则可省去 if 并把 should,had,were 移到句首，构成倒装句式，而意思不变。
3.【答案】B
【解析】此句考点是虚拟语气。及物动词advise, agree, ask, beg, command, decide, demand, determine, dictate等后面的宾语从句须用虚拟语气。虚拟语气的形式是should+动词原形,should可以省去，因此选B。
4.【答案】C�
【解析】if only引导的句子用虚拟语气，表示与过去的事实相反时，动词用过去完成时。
5.【答案】D。
【解析】当主句与从句时态不一致时，主句与从句分别使用与各自时态相应的虚拟语气。此句中从句与过去事实相反，主句与现在事实相反，因而选项D正确。
6.【答案】C。
【解析】 在It is/was urgent that…句型中，that引导的从句谓语动词应用“should+动词原形”形式，should可以省略。类似的形容词还有necessary, advisable, desirable, vital等。
7.【答案】B。�
【解析】 It's high time...结构后应使用虚拟语气，something在这里应接过去时态单数谓语动词was。故选B。
8.【答案】A。
【解析】 “insist”一词意思是“坚决要求” 时，后面的句子用虚拟语气，即宾语从句的谓语用(should)+动词原形。
9.【答案】C。
【解析】 if 条件句中包含有 should, had,were 则可省去 if 并把 should,had,were 移到句首，构成倒装句式，而意思不变。
10.【答案】D。
【解析】此题中前半句是一个陈述句，后半句中otherwise相当于一个虚拟的条件从句“If the traffic had not been very busy”，是对过去的情况的一个假设，因此主句中的谓语要用“would have done”的形式。
参考资料：http://edu.sina.com.cn/exam/2006-10-08/091256172.html