含有虚拟条件句的复合句中谓语动词构成的基本形式：
例如：
If everyone in the country bought one soft-drink each day and threw it away, there would soon be a huge mountain of rubbish. 如果全国每个人每天买一罐软饮料，然后把空罐扔掉，那么垃圾很快就会堆积如山了。（与现在事实相反）
If the hurricane had happened during the day time, there would have been many more deaths. 倘若飓风发生在白天，那么死亡的人数还会多得多。（与过去事实相反）
If it were to snow tomorrow, they would not go out. 如果明天下雪，他们就不出去了。（与将来事实相反）
【考例】 If the new safety system_______to use, the accident would never have happened.（2017北京卷）
A. had been put
B. were put
C. should be put
D. would be put
【答案】 A
【解析】 根据句子结构以及句中的信息would never have happened可知，if引导的是虚拟条件句，表示对过去的虚拟，从句应使用过去完成时。句意为：如果新的安全系统已经投入使用的话，那场事故就不会发生了。
【考例】 Sorry, I am too busy now. If I_______time, I would certainly go for an outing with you.（2012湖南卷）
A. have had
B. had had
C. have
D. had
【答案】 D
【解析】 句意为：抱歉，我现在太忙了。如果有时间，我一定和你去郊游。根据第一句话中的时间状语now可知此处谈论的是现在的事情。在后面的主从复合句中，主句中谓语是“would+动词原形”，但谈论的是现在的事情，由此可知该句使用了虚拟语气。根据主句中的动词形式可知从句中的假设与现在事实相反，所以从句动词应使用过去时。
以下几种特例应引起同学们注意：
如果在条件句中含有were, had或should，可将if省略，然后将were, had或should移至主语之前。例如：
Should there be a meeting tomorrow, I would come. 如果明天有会议召开，我会来的。
Had we made a great effort, we might have succeeded. 如果我们（过去）做了很大的努力的话，我们可能就获得成功了。
【考例】 ______ not for the support of the teachers, the student could not overcome her difficulty.（2017江苏卷）
A. It were
B. Were it
C. It was
D. Was it
【答案】 B
【解析】 句意为：如果没有老师们的帮助，这个学生就克服不了她的困难。根据语境可知，此处考查的是虚拟语气，并且条件句中省略了if，应使用倒装语序，将were提到句首，故选B项。
【考例】 Had the governments and scientists not worked together, AIDS-related deaths_______since their highest in 2005.（2016浙江卷）
A. had not fallen
B. would not fall
C. did not fall
D. would not have fallen
【答案】 D
【解析】 句意为：如果没有政府和科学家的共同努力，同艾滋病相关的死亡人数是不会从2005年的最高点回落的。由从句中的“Had the governments and scientists not worked together”可知，此处是省略了 if的虚拟条件句，且是对过去状况的虚拟，所以主句中谓语动词应使用would have done。
有时条件句的动作与主句动作发生的时间不一致，这时动词的形式应根据它们表示的时间加以调整。例如：
If it had rained last night, it would be very cool today. 如果昨天晚上下了雨的话，今天就会非常凉爽了。
【考例】 If it_______for his invitation the other day, I should not be here now.（2016江苏卷）
A. had not been
B. should not be
C. were not to be
D. should not have been
【答案】 A
【解析】 句意为：要不是因为他那天的邀请，我现在就不会在这里了。根据句意可知本题考查混合虚拟语气。从句与过去(the other day)的事实相反，而主句与现在(now)的情况相反，所以从句用过去完成时表示对过去的虚拟，故选A项。
（1）用连词otherwise, or, but 等暗示前面的条件。例如：
I thought you would remain there for a while after the conference. Otherwise I wouldn't have bought you a single ticket. 我原以为会后你要在那儿逗留一段时间。要不然我就不会给你买单程票了。
Mary could be prettier, but she doesn't care much about her clothes. 玛丽本可以更漂亮些，但是她不太在乎衣着。
【考例】 —Do you have Betty's phone number?
—Yes. Otherwise, I_______able to reach her yesterday.（2017天津卷）
A. hadn't been
B. wouldn't have been
C. weren't
D. wouldn't be
【答案】 B
【解析】 根据对话的语境可知，此处使用了虚拟语气。根据时间状语yesterday可知是对过去的虚拟。Otherwise暗示条件，空格处应为主句谓语，所以应选B项。
（2）用but for, without等介词短语表达条件。例如：
Without electricity, human life would be quite different today. 如果没有电，人类现在的生活就会是另一个样子了。
The ship would have sunk with all on board but for the efforts of the captain. 要不是因为船长的努力，连船带人都已沉没了。
【考例】 Without his wartime experiences, Hemingway_______his famous novel A Farewell to Arms.（2015重庆卷）
A. didn't write
B. hadn't written
C. wouldn't write
D. wouldn't have written
【答案】 D
【解析】 句意为：要是没有战争年代的经历，海明威就不会写出他的著名小说《永别了，武器》。without短语表达含蓄的条件，暗示这是一个含蓄条件句，后面是主句。该含蓄条件表示的是对过去的虚拟，故主句应与过去事实相反，应使用“would / should / could /might...+have done”，故选D项。
【考例】 But for their help, we_______the program in time.（2009安徽卷）
A. can not finish
B. will not finish
C. had not finished
D. could not have finished
【答案】 D
【解析】 根据But for their help可以推断这是一个与过去事实相反的含蓄虚拟条件句，主句动词应使用“would / could / might / should...+完成时”形式，故D项为正确选项。
（3）通过分词短语表示条件。例如：
Given more time, we could have done it better. 假如（过去）多给一些时间的话，我们就能把它做得更好一些了。
（4）用动词不定式表示条件。例如：
It would be a mistake not to help him. 倘若不帮助他，那会是一个错误。
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