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语气用来表示说话者所持的态度或者说话的意图。不同的态度或意图要用不同的语气来表示，英语中的语气分为四种：陈述语气、疑问语气、祈使语气和虚拟语气。
在这四种语气中，虚拟语气属于特殊的语气，主要用来表示一种非真实的假设。高考中主要考查虚拟语气表示非真实条件句的用法以及名词性从句（宾语从句、主语从句、表语从句和同位语从句）中的虚拟情况。
if所引导的表示“如果”含义的状语从句是典型的条件状语从句，分为真实条件句和非真实条件句两种：真实条件句表达可能实现的条件；非真实条件句则表达不可能实现或者很难实现的条件。
非真实条件句应采用虚拟语气，具体分为三种情况：与现在事实相反；与过去事实相反；与将来事实相反。下表总结了具体的表现形式：
以下为各种非真实条件句的具体例句：
（1）与现在事实相反的非真实条件句
①If I were invited, I might go to her wedding today. 如果我被邀请，今天我可能会去参加她的婚礼。 （实际情况是：她没有邀请我，我没去参加她的婚礼。）
②If Xiao Zhang were here, she might help us. 如果小张在这儿的话，她可能会帮助我们。 （实际情况是：小张没有在这儿，她无法帮助我们。）
（2）与过去事实相反的非真实条件句
①If he had studied hard at other times, he should have passed the exam. 如果他平时学习努力的话，他考试就通过了。 （实际情况是：他平时学习不努力，因此考试未通过。）
②If I had taken his advice, I wouldn't have made such a mistake. 如果我采纳了他的忠告，我就不会犯这样的错误了。 （实际情况是：我没有采纳他的忠告，我犯了错误。）
（3）与将来事实相反的非真实条件句
①If he were not to start early tomorrow, he could not arrive on time. 如果他明天不早点出发的话，他就不能按时到达。 （实际情况是：明天他会早点出发。）
②If it should rain tomorrow, the basketball match would be put off. 如果明天下雨的话，篮球比赛就得延期。 （实际情况是：明天下雨的可能性不大。）
语法考点运用实例·典型单选题>>
【例1】 Sorry, I am too busy now. If I______time, I would certainly go for an outing with you.
A. have had B. had had C. have D. had
【答案】 D
【解析】 下划线处显然是针对现在情况的虚拟，由于主句采用的是“would＋动词原形”的形式，所以 if 从句中应采用一般过去时的形式。
【例2】 If we______adequate preparations, the conference wouldn't have been so successful.
A. haven't made B. wouldn't make C. didn't make D. hadn't made
【答案】 D
【解析】 题干句意为：如果我们没有做好充分的准备，会议就不会如此成功。“wouldn't have been so successful”是表示与过去事实相反的虚拟语气形式，if从句中的谓语动词应采用“had+过去分词”的形式。
非真实条件句在高考中还经常以特殊形式出现，主要包括以下三种情况：
（1）倒装条件句。当非真实条件句中含有were、had、should 等系动词或助动词时，可省略if，把系动词或助动词放在非真实条件句的句首，呈现倒装形式。这种结构主要用于书面语。例如：
Were they here now, they could help us. =If they were here now, they could help us. 如果他们现在在这 儿，他们就可以帮助我们。
Had you come earlier, you would have met him. =If you had come earlier, you would have met him. 你早来一点儿的话就可以遇见他。
Should it rain, the crops would be saved. =If it should rain, the crops would be saved. 如果下雨的话，这些农作物将获救。
（2）含蓄条件句。含蓄条件句指非真实的假设并不通过if引导的状语从句表现，而利用其他手段的情况。含蓄条件句往往都可以转化为if引导的条件句，在高考中主要有以下几种具体形式：
①用without、but for等介词（短语）表示条件。例如：
Without air and water (=If there were no air and water), all living things on the earth would die. 地球上如果没有空气和水，一切生物都将死亡。
But for the car accident (=If there had not been the car accident), we would have arrived there much earlier. 要不是因为车祸，我们早就到了。
②用分词短语表示条件。例如：
Given more time (=If we had been given more time), we could have done the task much better. 如果再给我们点时间，我们可以把任务完成得更好。
③用连词or、otherwise、but、once、though等表示条件。例如：
He was taken to hospital at once yesterday, otherwise/or he would have died already. =If he had not been taken to hospital at once yesterday, he would have died already. 昨天他被立即送到了医院，要不然他早就死了。
I would have attended the important meeting, but I was too busy then. =If I had not been too busy then, I would have attended the important meeting. 要不是当时太忙，我就参加那个重要会议了。
Einstein cared little for money, though he could have been very rich. =If Einstein had cared much for money, he could have been very rich. 如果爱因斯坦很在乎钱的话，他已经很富有了。
Once lost, it would be very hard to find again. =If it were once lost, it would be very hard to find again. 一旦失去了，就不容易再找回。
（3）错综时间条件句。如果非真实条件状语从句中谓语动词所针对的时间与主句中谓语动词所针对的时间不一致，就属于错综时间条件句（又称混合条件句）。主要有以下几种不同形式：
①从句针对过去，主句针对现在。例如： If you had studied hard before, you would pass the test. 如果你以前学习努力的话，你现在就通过考试了。
条件句针对过去，因此谓语动词用“had＋过去分词”的形式；主句针对现在，因此谓语动词用“would＋动词原形”的形式。
②从句针对过去，主句针对将来。例如： If we hadn ' t made adequate preparations, we shouldn ' t dare to do the experiment next week. 如果我们没有做好充分的准备，我们将不敢在下周做这个实验。
③从句针对现在，主句针对将来。例如： If they started now, they would be here tomorrow. 如果他们现在动身的话，明天他们都能在这了。
④从句针对将来，主句针对过去。例如： If I were not to make a preparation for my experiment this afternoon, I would have gone to see the film with you last night. 如果我不是要为我今天下午的实验做准备，我昨晚就陪你看电影去了。
⑤从句针对将来，主句针对现在。例如： If we shouldn ' t have an exam this afternoon, I would go shopping now. 如果我们今天下午没有测验，我现在就购物去了。
【例3】 I______sooner but I didn't know that they were waiting for me.
A. had come B. was coming C. would come D. would have come
【答案】 D
【解析】 “but I didn't know that they were waiting for me”暗含着与过去事实相反的非真实条件句，即“if I had known that they were waiting for me”，所以前面下划线处应采用相应的虚拟语气形式。
【例4】 —Pity you missed the lecture on nuclear pollution.
—I______it, but I was busy preparing for a job interview.
A. attended B. had attended C. would attend D. would have attended
【答案】 D
【解析】 题干中“but I was busy preparing for a job interview”意味着说话人当时较忙、难以抽时间去听演讲，暗含着非真实条件句“if I had not been busy”，因此前面下划线处应采用相应的虚拟语气形式。
【例5】 It was announced that only when the fire was under control______to return to their homes.
A. the residents would be permitted B. had the residents been permitted C. would the residents be permitted D. the residents had been permitted
【答案】 C
【解析】 本题既考查虚拟语气，又考查倒装句的用法，题干中的it作形式主语，that 引导的真正主语当中only与时间状语位于句首，其后需要以倒装句的形式进行表述，考生可以先排除选项A、D；由于下划线处针对“when the fire was under control”这样的在将来可能出现的情况构造虚拟语气，考生可以进一步排除选项B而选出C。
名词性从句包括宾语从句、主语从句、表语从句和同位语从句四种。关于名词性从句本书第十七章将有专门讲解，此处我们只重点介绍这四种从句中所涉及的虚拟语气。
如果这四种从句表达的是一种不太可能实现或者陈述人自身就没有想过会实现的愿望、意图，就适宜采用虚拟语气，谓语动词通常要使用could、would、should等情态动词，例如： I wish I could fly.我希望我会飞。
如果这四种从句表达的是一种建议或者惊奇、不相信、惋惜、理应如此等强烈语气，也适宜采用虚拟语气，谓语动词通常用“should +动词原形”的形式，其中should常常被省略。例如： I suggest that you (should) spend your holiday in beautiful European countryside. 我建议你去美丽的欧洲乡村度假。// My suggestion is that you (should) spend your holiday in beautiful European countryside. 我的建议是你去美丽的欧洲乡村度假。// His suggestion that you (should) spend your holiday in beautiful European countryside sounds reasonable. 他提出的你去美丽的欧洲乡村度假的建议听起来有道理。// It is suggested that you (should) spend your holiday in beautiful European countryside. 有人建议你去美丽的欧洲乡村度假。 以下主语从句句型中也常使用虚拟语气：It is necessary/important/natural/strange that...。
常见的名词性从句中使用虚拟语气的情况还有表示“宁愿”之意的would rather所接的宾语从句，其谓语动词要用动词的过去式。例如： I would rather that you spent your holiday in European countryside. 我宁愿你是去美丽的欧洲乡村度假。
语法考点运用实例·典型单选题>>
【例1】 Teachers recommend parents_______their children under 12 to ride bicycles to school for safety.
A. not allow B. do not allow C. mustn't allow D. couldn't allow
【答案】 A
【解析】 题干句意为：老师们向家长们建议，不要让他们不满12岁的孩子骑自行车上学以保证孩子们的安全。本题考查的是宾语从句中的虚拟语气，recommend意为“建议”，后面接了省略连接词that的宾语从句，此时从句中需要使用虚拟语气，谓语动词的形式应为：should + 动词原形（should可以被省略）。
【例2】 Jack is a great talker. It's high time that he______something instead of just talking.
A. will do B. has done C. must do D. did
【答案】 D
【解析】 在句型“It's high/very time that...（恰恰该是……的时候了）”中，名词性从句中的谓语动词通常用should do或过去式的形式。
本章补充强化训练题>>
1. The boy would have died,______on him without delay.
A. if the doctor didn't operate B. if the doctor wouldn't operate C. would the doctor not operate D. had the doctor not operated
2. It has been raining for a day, but even though it______, we______there by tomorrow.
A. hadn't rained; couldn't get B. hadn't rained; can't get C. didn't rain; couldn't get D. didn't rain; can't get
3. If places______alike, there would be little need for geographers.
A. being B. are C. be D. were
4. If I______the meaning of the word, I wouldn't have to look it up.
A. know B. knew C. known D. knowing
5. We didn't know his telephone number, otherwise we______him.
A. would have telephoned B. must have telephoned C. would telephone D. had telephoned
6. 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. had got C. were to get D. could have got
7.______I known it, I would have told you.
A. Have B. If C. Had D. Having
8. Had he worked harder, he______the exams.
A. must have got through B. would have got through C. would get through D. could get through
9. After he was praised for what he had done, he said,“I______even better under harder conditions.”
A. would do B. would have done C. did D. had done
10. We could not have succeeded______your help.
A. for B. without C. if it not been D. all with
本章补充强化训练题参考答案>>
1. D 2. B 3. D 4. B 5. A 6. C 7. C 8. B 9. B 10. B