条件从句有两类，一是真实条件句，另一是虚拟条件句。如果假设的情况可能发生，是真实条件句，这种情况下谓语用陈述语气。如：
If time permits, we’ll go fishing together.如果时间允许，我们就一起去钓鱼。
如果假设的情况是不存在的或不大可能发生的，则是虚拟条件句。如：
If you had come yesterday, you would have met that famous professor.如果你昨天来，你就会见到那位著名的教授了。（隐含的事实是：你昨天没来，也没见到那位著名教授）。 一、虚拟语气的基本形式和用法
在含有虚拟条件句的复合句中，主从句的谓语都要用虚拟语气，现将其主句与从句中谓语动词的形式列表如下：
假设类型 与现在事实相反 条件从句谓语动词形式 If+主语+动词的过去式 (be用were而不用was) If+主语+had+-ed分词 If+主语+动词过去式 主句谓语动词形式 主语+would(第一人称可用should)+动词原形 主语+would(第一人称可用should)+have+过去分词 主语+would(第一人称可用should)+动词原形 与过去事实相反 与将来事实可能相反 If+主语+were to+动词原形 If+主语+should+动词原形 1．表示与现在事实相反的假设和结果。如：
If I were you, I should(would ,could, might)tell him the truth. 要是我是你，我就会告诉他真相了。（事实上我不是你）
If she had time, she would(could, might)help me.如果她有时间，她就会帮我了。（事实上她没有时间）
2 表示与过去事实相反的假设和结果。如： If I had known your telephone number yesterday, I
should(would, could, might)have telephoned you.如果昨天我知道了你的电话号码，我就会给你打电话了。（事实上我昨天不知道你的电话号码。）
If you had taken my advice, you wouldn’t(couldn’t) have failed in the exam.如果你听了我的建议，你就不会考试不及格。（事实上你根本没听我的。 3 表示与将来事实可能相反的假设和结果。如：
If it should rain, the crops would(could, might)be saved.假如天下雨，庄稼可能就收获了。
---What would happen if I put the paper on the fire?
---It would burn.
---我若把只放在火上会怎么样？ ---纸会烧着。
注意问题：
1、If条件句中绝对不可出现“would”。
2、主句中的should只用于第一人称，但在美国英语中，should常被would代替；从句中的should可用于各种人称。
3、在表示与将来事实可能相反的条件从句中，were to + 动词原形比较正式，常用于书面语中。如：
If you were to go to Beijing, you would (could, might) have a chance to visit Tian An Men Square. 4、在if引导的表示虚拟的条件状语从句中，在条件句中如果出现were, had, should可省去if，将had, were或should提至句首。如： 例如：
If the doctor had come last night, the boy would have saved. Had the doctor come last night, the boy would have saved.
If I were to go to the moon one day, I would see it with my own eyes. Were I to go to the moon one day, I would see it with my own eyes. If it should rain tomorrow, we would stay at home. Should it rain tomorrow, we would stay at home.
Had I seen the film, I would have discussed it with them last night. 假如我看了那部电影，昨晚我就可以和他们一起讨论了。
Were I a bird, I could fly freely. 假如我是一只小鸟，我就能自由翱翔。 Should it rain next week the farmers would have a good harvest. 要是下周能下雨的话，农民们就能有个好收成了。
★根据句中的时间状语，有时可能出现“混合虚拟”的情况，即主句可能是现在的情况，条件句也许是发生在过去的情况，但都是遵守上述句型的形式。 主从句时间不一致情况下的虚拟语气(混合条件句) 有时条件从句中的动作和结果与主句中的动作，发生的时间不一致，这时动作的形式应根据它所表示的时间加以调整。如：
If you had followed my advice, you would be able to finish the work now. 如果你当时听了我的话，现在就能完成这份工作了。（从句说明过去，主句说明现在。）
If I were you, I would have gone to her birthday party. 如果我是你，我就去参加她的生日晚会了。（从句说明现在，主句说明过去。） If you hadn’t lent me some money, I couldn’t have bought the new house and most likely I would be still living in the dangerous house now.
假若你不借钱给我，我不可能买下这幢新房，很可能现在还住在危房里。（从句说明过去，主句说明过去和现在。）
含蓄条件句
非真实条件句中的条件从句有时不表出来，只暗含在上下文中，这种句子叫做含蓄条件句。含蓄条件句大体有三种情况； 1.条件暗含在短语中。如：
(1)What would I have done without you?
如没有你，我会怎么办呢？(条件暗含在分词短语without you中) (2) It would be easier to do it this way.
这样做会比较容易。(条件暗含在不定式短语to do it this way中) (3) This same thing, happening in wartime, would lead to a disaster.
同样的事，如发生在战时，就会酿成大祸。(条件暗含在分词短语happening in wartime中)
(4) But for your help we couldn’t have succeeded in the experiment.
如果没有你的帮助，我们的实验是不会成功。 (暗含条件是but for your help) (5) He must have the strength of a hippopotamus, or he never could have vanquished that great beast.
他一定是力大如河马，否则他绝不会击败那只庞大的野兽。(暗含条件是连词or) (6) Alone, he would have been terrified.
如是单独一人，他是会感到害怕的。(暗含条件是alone) 2. 条件暗含在上下文中。如：
(7) You might stay her forever. 你可以永远待在这儿。(可能暗含if you wanted to) (8) We would have succeeded. 我们本来是会成功的。(可能暗含if we had kept trying)
(9) Your reputation would be ruined. 你的名誉会败坏的。(可能暗含if you should accept it)
(10) I would appreciate a little of your time.
谢谢你给我一点时间吧。(可能暗含if you were so kind as to give me a little of your time)
3. 在不少情况下，虚拟式已变成习惯说法，很难找出其暗含的条件。如： (11) You wouldn’t know. 你不会知道。 (12) I would like to come. 我愿意来。
(13) I wouldn’t have dreamed of it. 这是我做梦也不会想到的。
(14) He told the story in such minute detail that he might himself have been an eye-witness.
他将那事讲的非常仔细，简直就象他亲眼看见一样。
真题
例(1)Mary said to me, \"Had I seen your bag, I _____ it to you.\"
A. will return B. must return C. could return D. would have returned
(【答案】D)(2004年4月45题)
例(2)I would ask George to lend us the money if I _____ him. A. had known B. have known C. knew D. know (【答案】C)(1995年38题)
例(3)If Bob____ with us; he would have had a good time.
A. would come B. would have come C. had come D. came (【答案】C)(1995年9题) 二、省略if的虚拟形式
在虚拟条件句中，如谓语包含were，had，should等词，则可以把这些词放到主语前面，省略if。
例(1)__________ the advice of his friends, he would not have suffered such a heavy loss in his business.
A. If he took B. If he should take C. Were he to take D. Had he taken (【答案】D)(2004年4月32题)
例(2)__________, I should ask them some questions. A. Should they come to us B. If they come to us C. Were they come to us D. Had they come to us (【答案】A)(1997年30题)
例(3)__________ you were busy, I wouldn't have bothered you with my questions. A. If I realized B. Had I realized C. Did I have realized that D. As I realized (【答案】B)(1996年39题) 三、含蓄条件句
有时一个假设的情况不用条件从句表示，而用其它方式来表示，这样的句子叫含蓄条件句。常用with，without，but for。
例(1)Without heat and sunlight, plants on the earth ______ well.
A. would not grow B. will not grow C. had not grown D. would not be grown (【答案】A)(2004年11月32题)
例(2)But for my classmates' help, I _____ the work in time.
A. did not finish B. could not finish C. will not finish D. would not have finished (【答案】D)(2004年4月36题)
例(3)But for the rain, we _____ a nice holiday.
A. should have B. would have had C. would have D. will have had (【答案】B)(2003年4月28题) 四、特殊词或结构中的虚拟语气：
① 以wish(that)引导的表示“愿望”的宾语从句
wish后的宾语从句中要用虚拟语气，而在虚拟语气中时态的应用类似在非真实条件中从句时态的应用。表示现在或将来的愿望用：主语＋wish ＋从句(主语＋过去时)；表示一个过去没有实现的愿望用：主语＋wish +从句(主语＋过去完成时)； 例(1)I didn’t go to the party, but I do wish I _____ there. A. were B. would be C. had been D. will be (【答案】C)(2001年53题)
例(2)Peter wishes that he _____ law instead of literature when he was in college. A. could study B. studied C. had studied D. would study (【答案】C)(2000年53题)
② if only +句子(过去时/过去完成时)“要是…就好了”
If only 后面的句子如果是对现在或将来情况的虚拟，用过去时did ；如果是对过去情况的虚拟，用过去完成时had done . 要注意与if 区别，if 只表示假设，“如果”，用真实语气；而if only 表示不可能实现的情况，用虚拟语气。
例如：If only the alarm clock had rung. 当时闹钟响就好了。(对过去虚拟)
If only I were rich. 要是我富有就好了。(对现在虚拟)
③ would rather＋句子(过去时) 表示宁愿、宁可的意愿，语气比较婉转。后接过去式。 例(1)I am too busy these days. I would rather all of you _____ next month for a dinner. A. come B. would come C. came D. have come (【答案】C)(2002年46题)
例(2)I’d _____ you didn’t touch that, if you don’t mind. A. rather B. better C. happier D. further (【答案】A)(1998年45题)
④ it is time(that)，it is high time (that)…句型中，谓语动词用过去时，表示早该做而未做的事，意思是该……了、早该……了.
例(1)Let's finish our homework in a few seconds; it's time we __________.
A. played football B. will play football C. play football D. are playing football (【答案】A)(2004年11月43题)
例(2)Don’t you think it is time you _____ smoking?
A. give up B. gave up C. would give up D. should give up (【答案】B)(1999年31题) ⑤ 以as if，as though引导的从句
在as if，as though引导的从句中，如果谈论的是不可能或不真实的情况时，它们所引用的状语从句要用虚拟语气，动词形式和wish后面的从句动词形式变化相同。表示现在或将来的愿望用：主语＋as if＋从句(主语＋过去时)；表示一个过去没有实现的愿望用：主语＋as
if +从句(主语＋过去完成时)；
例：He talks as if he _____ everything in the world.
A. knows B. knew C. had known D. would have known (【答案】B。本句表示说话人对现在的情况有所怀疑，所以用过去时。) 五、should+动词原形及should的省略
(1)、一些表示建议、要求、命令等的动词本身隐含说话者主观的意见。因此这些动词(或其变形，如名词，形容词，分词等)后面的从句应使用虚拟语气，即“should do”结构，其中的“should”常被省略。如：
表“提议、建议”的动词：suggest ,advise, propose, recommend, move(提议); 表“要求、主张”的动词：insist, require, request, demand , desire ; 表“命令”的动词：order, command.
例(1)I don’t think it advisable that Tom _____ to the job since he has no experience. A. be assigned B. is assigned C. will be assigned D. has been assigned (【答案】A)(2006年11月29题)
例(2)His mother insisted that he _____ the coat when going out. A. put on B. puts on C. to put D. putting on (【答案】A)(1999年58题) 此处省略了should.
例(3)The doctor advised that Mr. Malan _____ an operation right away so as to save his life.
A. had B. would have C. have D. was going to have (【答案】C)(1998年28题) 此处省略了should.
(2)、It is necessary/important/essential/ vital/urgent / surprising/ amazing that引导的主语从句中，从句的谓语动词要用should+原形动词，should也可以省略。此类形容词的特点是表示重要性或吃惊。
例(1)it is vital that enough money_____collected to get the project started. A. is B. be C must be D can be (【答案】B)(2004年11月24题)
例(2)It’s urgent that a meeting _____ before the final decision is made.
A. will be arranged B. must be arranged C. be arranged D. would be arranged (【答案】C)(2003年4月45题)
(3)、以in case, lest, for fear that引起的状语从句，表示“以防，以免”。其形式为should+动词原形。如：
He took his raincoat with him in case it should rain. 他把雨衣带在身上以防下雨。
1． Elize remembers everything exactly as if it _______yesterday.(2006全国卷一) A. was happening B .happens C. has happened D. happened
解析:D. 考察虚拟语气.as if 引导的从句通常用虚拟语气.又因 remember知 与现在事实相反,因此用一般过去时.
2. _____fired, your health care and other benefits will not be immediately cut off. (2006湖北)
A. would you be B. should you be C. could you be D. might you be
解析:B考察虚拟语气的倒装.if 引导的条件状语从句,如果用虚拟语气,可以把if 省略,将助动词were, had 或should 提前,构成倒装.
3. I was on the highway when this car went past followed by a police car. They ____at least 150 kilometers an hour.(2005 重庆)
A. should have been doing B. must have been doing C. could have done D. would have done 解析:B 考察虚拟语气.表推测
4. He paid for a seat, when he ____have entered free.(2005 山东) A. could B. would C. must D. need 解析: A. 考察虚拟语气情态动词的用法
5. Mr.White ______at 8:10 for the meeting, but he didn’t show up.(2004全国卷三) A. should have arrived B. should arrive
C. should have had arrived D. should be arriving
解析: A 本题考察虚拟语气及责备的用法. Should have done 结构,表过去原本该做而没做的事.
高考预测：
1. What would have happened if you____her child ?
A.hadn’t helped B. couldn’t help C. wouldn’t help D. didn’t help
解析：A表示过去时间的虚拟是条件从句，用过去完成时，主句用would (should)+have + 动词原形。
2. If I ___with her last summer, I ___with her now . A. worked ---am getting on very well B. had worked ---will get on very well C. had worked ---would have got on very well D. have worked ---will get on very well.
解析：B。本题是混合式虚拟语气。主句是对现在时间的虚拟。 3. The boss ordered that he ___the work at once A. starts B. will start C. would start D. start
解析：D order, suggest 等词后的宾语从句，要用虚拟语气，构成为should +v, should可以省略。
4. The little hero’s pale face suggested he ________dead. A. is B. was C. should be D. be
解析；B 不作建议讲时，宾语从句谓语不用虚拟语气。 5._________I known it, I should have told him . A. have B. had C. having D. If
解析：B 这是一个与过去事实相反的虚拟语气，如非真实性条件句谓语含had ,should ,were 时，可将这些词提到句首，省去if.
6.If you had gone to a doctor, you ________all right now. A. would not come B. might not come c. should not come D. could not come
解析：C 表示与将来事实相反的虚拟语气句。
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