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虚拟条件句是中学英语课本中的一个重点句法，也是历年高考的热点。在此将这一句法的基本用法及其在句中的活用分析如下：
一、虚拟条件句的基本结构及其用法
1.与现在事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合现在的实际情况，主从句必须使用虚拟语气。其结构为：If sb. did (were)…, sb. would (should, could, might) + 动词原形。（注意：从句中的动词were可用于各种人称，主句中的should只可用于第一人称单复数。）
例如：If I wereyou, I would not doit.
If he hadtime, he would goto the concert with you.
If itwerenot for the expense, I would goto Italy.
2．与过去事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合过去的实际情况，主从句必须使用虚拟语气。其结构为：If sb. had done sth, sb. would (should, could, might) + 动词完成式( have done sth. )。（注意：在这一结构中，如果从句中有过去时间状语，其谓语动词不能用一般过去时）。
例如：If my lawyer had beenhere last Saturday, he would have preventedme from going there.
If he had taken my advice, he would not have failde in the exam.
If it hadn’t beenfor your help, I couldn’t have finishedit ahead of time.
高考链接
1) ---If he ______, he _______that food.
---Luckily he was sent to the hospital immediately. (93′全国)
A. was warned, would not take
B. had been warned, would not have taken
C. would be warned, had not taken
D. would have been warned, had not taken
评析：选B。从答句Luckily he was sent to the hospital immediately谓语动词一般过去时的使用，说明选项动作也发生在过去。分析选项部分句意可知，它是表示与过去事实相反的一种假设，根据与过去事实相反的虚拟条件结构，从句用过去完成时，主句用would have done…。
2) You didn’t let me drive. If we _______ in turn, you ________so tired. (96′全国)
A.drove; didn’t get
B. drove; wouldn’t get
C.were driving; wouldn’t get
D. had driven; wouldn’t have got
评析：选D。从题干中前一分句You didn’t let me drive可知，讲话人所陈述事实发生在过去。分析后一分句条件状语从句意义可知，这是一个与过去事实相反的假设，说明“我们没有轮流开车，所以，你很疲劳”。根据所提供的四个选项，只有答案C符合句子结构。
3．与将来事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合将来的实际情况，主从句必须使用虚拟语气。其结构为：If sb. did (were to /should do) sth.…, sb. would (should, could, might) + 动词原形。（注意：从句中的should或were可用于各种人称，从句中无论哪种人称都不可用would。
If I should see him this afternoon, I would give the note to him.
If there were to bea meeting this evening, I would come.
高考链接
If it _______ tomorrow, we________ put off the visit to the Yangpu Bridge. (94′上海)
A.rains; would have had to
B.were to rain; will have to
C.should rain; would have to
D.rained; should have had to
评析：选C。根据语境中所给时间状语tomorrow可知，这是表示将来的条件状语从句，本句可使用陈述语气（条件句用一般现在时，主句用一般将来时），也可使用与将来事实相反的虚拟语气（条件句用did/were to do/should do…，主句用would do…），根据所给选项，只有答案C为正确答案。
二、虚拟条件句的活用
1．时间错综虚拟结构
在虚拟条件句中，有时主句与从句所指的时间不一致，这时，我们不能照搬虚拟条件句式结构的某种形式，而应根据主从句动词实际的时间概念来确定动词的虚拟结构。
例如：If she had followed the doctor’s advice, she would be quite all right now.
如果她当初听了医生的忠告的话，她现在就完全康复了。[说明；这是一个与事实相反的非真实条件句。if引导的条件句中的动作是过去发生的事情，说明与过去事实相反，应选用过去完成时，而主句中的动作受现在时间状语now限制，说明与现在事实相反，故主句应选用would(could, should, might) + 动词原形结构。]
如：If the young tree had been watered, it wouldn’t die now.
如果给那棵小树浇了水，它现在就不会枯死了。
高考链接
It is hard for me to imagine what I would be doing today if I _______ in love, at the age of seven, with the Melinda Cox Library in my hometown. (02′上海)
A.wouldn’t have fallen B. had not fallen
C.should fall D. were to fall
评析：选B。根据题干中what I would be doing today可知，这是表示与现在事实相反的主从结构，而if引导的条件状语从句中所给的时间状语为at the age of seven过去时间状语，故从句应选用与过去事实相反的虚拟结构（过去完成时）。
2．省略if的虚拟结构
在虚拟条件句中，如果if 引导的虚拟条件句中有should, had,或were等助动词，表示与将来、过去或现在事实相反，if常可以省略。这时，从句要采用倒装结构，即将从句中助动词should, had, were调至主语之前。
例1. Were Iin school again, I would studyharder. = If I werein school again, Iwould studyharder.
如果我（现在）再上学的话，我会更加努力学习的。(与现在事实相反的虚拟条件句)
例2.Should it (Were it to) snowtomorrow, we would be very happy. =If it should (If it were to) snow tomorrow, we would be very happy.
如果明天下雪，我们将很高兴。(与将来事实相反的虚拟条件句)
例3.Had it not been foryour help, Icouldn’t have finishedit ahead of time.= If it hadn’t been for your help, I couldn’t have finished it ahead of time.
要不是你的帮助，我就不能提前完成这项任务。(与过去事实相反的虚拟条件句)
高考链接
1.______for the free tickets, I would not have gone to the films so often. (’95 上海)
A. If it is not B. Were it not
C. Had it not been D. If they were not
评析：选C。根据主句中谓语动词结构would not have gone，说明前一分句是与过去事实相反的虚拟条件句，应选用过去完成时结构，分析四个选项，只有选项C（倒装结构）为正确答案。实际上，相当于一个陈述结构：If it had not been for…。
3．并列连词或副词暗示的虚拟结构
在一个并列句型结构中，有时假设的条件不通过条件从句表达出来，而是由句中的并列连词或副词暗示出来。常见的并列连词或副词有but, or, otherwise等。
例1．I would have gone to America with my friend, but I was stoppedby my mother.
要不是我母亲阻止我，我就跟我朋友去美国了。
评析：该句是由并列连词but连接的并列句。后一分句强调的过去发生的客观事实，故要用一般过去时的陈述结构；前一分句中，根据but转折连词的暗示，实际上省略了一个与过去事实相反的虚拟条件句If I hadn’t stopped，故前一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have gone。
如：I would have gone to your party, but you didn’t invite me.=I would have gone to your party (if you had invited me), but you didn’t inviteme.
例2. Yesterday he didn’t join inour discussion, or I would have toldhim about it.
昨天他没有参加我们的讨论，否则我就把这件事告诉他了。
评析：前一分句为过去已发生的客观事实，应选用一般过去时结构。但连词or暗示了后一分句中的一个与过去事实相反的非真实情况，实际上省略了一个与过去事实相反的条件句If he had joined in our discussion yesterday，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have told。
如：Iwas illthat day, or I would have taken part in the sports meet.=I was illthat day, or I would have taken partin the sports meet (if I hadn’t been ill that day)
例3．He didn’t get up earlier this morning. Otherwise, he would have caughtthe first bus.
今天早晨他起床迟了，要不然，他就赶上早班车了。
评析：句中副词otherwise暗示了条件，并说明了后一分句中的一个非真实情况。根据前一分句所表示的在过去所发生的客观事实情况，后一分句实际上省略了一个与过去事实相反条件句If he had got up earlier this morning，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have caught。
如：He remainedvery calm, otherwise he would have been found out.=He remainedvery calm, otherwise he would have been found out(if he hadn’t remainedvery calm).
高考链接
Yesterday Jane walked away from the discussion. Otherwise, she _______something she would regret later. (’96 上海)
A. had said B. said
C. might say D. might have said
评析：选D。分析题干，前一分句暗示了过去的事实，而句中副词otherwise暗示了条件，并说明了后一分句中的一个非真实情况，表示与过去事实相反。后一分句实际上省略了一个与过去事实相反的条件句If she had walked away from the discussion，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应选用might have said。
4．介词或介词短语暗示的虚拟结构
在一个简单句式结构中，有时假设的条件不通过条件从句直接表达出来，而隐含在某些介词短语中。常见的介词或介词短语有without, but for等。
例1．Without electricity, human life would be quite difficult today.
如果没有电，今天人类的生活就完全不同了。
评析：该句为一简单句，句中介词短语Without electricity暗示条件，实际上相当于一个明确的条件状语从句If there were no electricity。根据句意及是句中时间状语today说明句子是对现在的假设，所以，该句应是与现在事实相反的动词虚拟形式。
如：Without his advice, I couldn’t have doneit all by myself.=If it hadn’t been for his advice, I couldn’t have doneit all by myself.
如果没有他的建议，我就不可能独自做好这一切。（句中动作与过去事实相反）
例2．But for your help, we wouldn’t have succeeded in doing this experiment.
要不是你的帮助，我们就不能成功地做好这个实验。
评析：该句与上例类似，为一简单句结构。句中介词短语But for your help暗示条件，实际上相当于一个明确的条件状语从句If it hadn’t been for your help。尽管句中没有明确的时间状语，但根据句意可知，这个句子是对过去情况的假设，故句中的谓语动词应用与过去事实相反的虚拟结构形式。
如：But for the meeting, I would have goneto your party last Saturday.=If it hadn’t been for the meeting, I would have goneto your party last Saturday.
要不是那个会议，我就参加了你上周六举行的晚会了。（句中动作与过去事实相反）
高考链接
Without the air to hold some of the sun’s heat, the earth at night______, too cold for us to live. （‘97 上海）
A.would be freezing cold
B. will be freezing coldly
C.would be frozen cold
D. can freeze coldly
评析：选A。句中介词短语Without the air to hold some of the sun’s heat暗示条件，实际上相当于一个明确的条件状语从句If there were no air to hold…。根据句意，句中时间状语today说明句子是对现在的假设，所以，该句应用与现在事实相反的动词虚拟结构形式。
5．分词短语暗示的虚拟结构
在一个简单句式结构中，有时句中假设的条件隐含在现在分词短语或过去分词短语之中。句子的谓语动词结构必须根据分词短语所假设的情况或句中的时间状语来判断和选用。
例如：Having known earlier, we would have stopped such a thing from happening again.
如果我们早点知道这件事的话，我们就会阻止它再次发生了。
评析：句中Having known earlier为现在分词短语作状语，暗示条件，实际上相当于一个条件状语从句If we had known earlier。分析句意，该句是对过去情况的假设，因此，句中的谓语动词应选用与过去事实相反的虚拟结构形式would have stopped。
如：Having been given more money, we would have been to Hongkong instead of Dalian.=If wehad been given more money, we would have been to Hongkong instead of Dalian.
如果我们有更多的钱，我们就去了香港而不是去大连了。（句中动作与过去事实相反）
Being your age, I would studyharder.=If I were your age, I would studyharder.
如果我是你这个年龄，我会更加努力学习的。（句中动作与现在事实相反）
6．上下文语境中的虚拟结构
在英语句式中，有时既没有时间状语暗示虚拟条件，也没有介词短语或分词结构来暗示条件，而其虚拟条件是由上下文表达出来。这时，我们只有通过分析上下文所给条件，才能弄清句中动词的虚拟结构。
例如： ---He would have failed in the experiment last time.
---Luckily he followed your advice.
评析：根据对话情景可知，应答句Luckily he followed your advice（很幸运他听了你的建议）实际上就是上一句讲话人暗示的条件。从应答句所给特定的动词时态（一般过去时）看，说明上一句中讲话人所说内容是对过去情况的假设，故暗示的条件句应为If he hadn’t followed your advice。由此可见，选项部分应为主句的谓语动词，与过去事实相反，应选用would have failed。
如：---I would have come to your birthday party last Sunday.
---Unfortunately, you were out on business.
= ---I would have come to your birthday party last Sunday (if I hadn’t beenout on business)
---Unfortunately, you were out on business.
---上个星期天（如果我没有出差），我就来参加了你的生日晚会了。
---很不幸，你出差了。
一、虚拟条件句的基本结构及其用法
1.与现在事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合现在的实际情况，主从句必须使用虚拟语气。其结构为：If sb. did (were)…, sb. would (should, could, might) + 动词原形。（注意：从句中的动词were可用于各种人称，主句中的should只可用于第一人称单复数。）
例如：If I wereyou, I would not doit.
If he hadtime, he would goto the concert with you.
If itwerenot for the expense, I would goto Italy.
2．与过去事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合过去的实际情况，主从句必须使用虚拟语气。其结构为：If sb. had done sth, sb. would (should, could, might) + 动词完成式( have done sth. )。（注意：在这一结构中，如果从句中有过去时间状语，其谓语动词不能用一般过去时）。
例如：If my lawyer had beenhere last Saturday, he would have preventedme from going there.
If he had taken my advice, he would not have failde in the exam.
If it hadn’t beenfor your help, I couldn’t have finishedit ahead of time.
高考链接
1) ---If he ______, he _______that food.
---Luckily he was sent to the hospital immediately. (93′全国)
A. was warned, would not take
B. had been warned, would not have taken
C. would be warned, had not taken
D. would have been warned, had not taken
评析：选B。从答句Luckily he was sent to the hospital immediately谓语动词一般过去时的使用，说明选项动作也发生在过去。分析选项部分句意可知，它是表示与过去事实相反的一种假设，根据与过去事实相反的虚拟条件结构，从句用过去完成时，主句用would have done…。
2) You didn’t let me drive. If we _______ in turn, you ________so tired. (96′全国)
A.drove; didn’t get
B. drove; wouldn’t get
C.were driving; wouldn’t get
D. had driven; wouldn’t have got
评析：选D。从题干中前一分句You didn’t let me drive可知，讲话人所陈述事实发生在过去。分析后一分句条件状语从句意义可知，这是一个与过去事实相反的假设，说明“我们没有轮流开车，所以，你很疲劳”。根据所提供的四个选项，只有答案C符合句子结构。
3．与将来事实相反的虚拟条件句
在条件状语从句中，如果假设的条件不能符合将来的实际情况，主从句必须使用虚拟语气。其结构为：If sb. did (were to /should do) sth.…, sb. would (should, could, might) + 动词原形。（注意：从句中的should或were可用于各种人称，从句中无论哪种人称都不可用would。
If I should see him this afternoon, I would give the note to him.
If there were to bea meeting this evening, I would come.
高考链接
If it _______ tomorrow, we________ put off the visit to the Yangpu Bridge. (94′上海)
A.rains; would have had to
B.were to rain; will have to
C.should rain; would have to
D.rained; should have had to
评析：选C。根据语境中所给时间状语tomorrow可知，这是表示将来的条件状语从句，本句可使用陈述语气（条件句用一般现在时，主句用一般将来时），也可使用与将来事实相反的虚拟语气（条件句用did/were to do/should do…，主句用would do…），根据所给选项，只有答案C为正确答案。
二、虚拟条件句的活用
1．时间错综虚拟结构
在虚拟条件句中，有时主句与从句所指的时间不一致，这时，我们不能照搬虚拟条件句式结构的某种形式，而应根据主从句动词实际的时间概念来确定动词的虚拟结构。
例如：If she had followed the doctor’s advice, she would be quite all right now.
如果她当初听了医生的忠告的话，她现在就完全康复了。[说明；这是一个与事实相反的非真实条件句。if引导的条件句中的动作是过去发生的事情，说明与过去事实相反，应选用过去完成时，而主句中的动作受现在时间状语now限制，说明与现在事实相反，故主句应选用would(could, should, might) + 动词原形结构。]
如：If the young tree had been watered, it wouldn’t die now.
如果给那棵小树浇了水，它现在就不会枯死了。
高考链接
It is hard for me to imagine what I would be doing today if I _______ in love, at the age of seven, with the Melinda Cox Library in my hometown. (02′上海)
A.wouldn’t have fallen B. had not fallen
C.should fall D. were to fall
评析：选B。根据题干中what I would be doing today可知，这是表示与现在事实相反的主从结构，而if引导的条件状语从句中所给的时间状语为at the age of seven过去时间状语，故从句应选用与过去事实相反的虚拟结构（过去完成时）。
2．省略if的虚拟结构
在虚拟条件句中，如果if 引导的虚拟条件句中有should, had,或were等助动词，表示与将来、过去或现在事实相反，if常可以省略。这时，从句要采用倒装结构，即将从句中助动词should, had, were调至主语之前。
例1. Were Iin school again, I would studyharder. = If I werein school again, Iwould studyharder.
如果我（现在）再上学的话，我会更加努力学习的。(与现在事实相反的虚拟条件句)
例2.Should it (Were it to) snowtomorrow, we would be very happy. =If it should (If it were to) snow tomorrow, we would be very happy.
如果明天下雪，我们将很高兴。(与将来事实相反的虚拟条件句)
例3.Had it not been foryour help, Icouldn’t have finishedit ahead of time.= If it hadn’t been for your help, I couldn’t have finished it ahead of time.
要不是你的帮助，我就不能提前完成这项任务。(与过去事实相反的虚拟条件句)
高考链接
1.______for the free tickets, I would not have gone to the films so often. (’95 上海)
A. If it is not B. Were it not
C. Had it not been D. If they were not
评析：选C。根据主句中谓语动词结构would not have gone，说明前一分句是与过去事实相反的虚拟条件句，应选用过去完成时结构，分析四个选项，只有选项C（倒装结构）为正确答案。实际上，相当于一个陈述结构：If it had not been for…。
3．并列连词或副词暗示的虚拟结构
在一个并列句型结构中，有时假设的条件不通过条件从句表达出来，而是由句中的并列连词或副词暗示出来。常见的并列连词或副词有but, or, otherwise等。
例1．I would have gone to America with my friend, but I was stoppedby my mother.
要不是我母亲阻止我，我就跟我朋友去美国了。
评析：该句是由并列连词but连接的并列句。后一分句强调的过去发生的客观事实，故要用一般过去时的陈述结构；前一分句中，根据but转折连词的暗示，实际上省略了一个与过去事实相反的虚拟条件句If I hadn’t stopped，故前一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have gone。
如：I would have gone to your party, but you didn’t invite me.=I would have gone to your party (if you had invited me), but you didn’t inviteme.
例2. Yesterday he didn’t join inour discussion, or I would have toldhim about it.
昨天他没有参加我们的讨论，否则我就把这件事告诉他了。
评析：前一分句为过去已发生的客观事实，应选用一般过去时结构。但连词or暗示了后一分句中的一个与过去事实相反的非真实情况，实际上省略了一个与过去事实相反的条件句If he had joined in our discussion yesterday，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have told。
如：Iwas illthat day, or I would have taken part in the sports meet.=I was illthat day, or I would have taken partin the sports meet (if I hadn’t been ill that day)
例3．He didn’t get up earlier this morning. Otherwise, he would have caughtthe first bus.
今天早晨他起床迟了，要不然，他就赶上早班车了。
评析：句中副词otherwise暗示了条件，并说明了后一分句中的一个非真实情况。根据前一分句所表示的在过去所发生的客观事实情况，后一分句实际上省略了一个与过去事实相反条件句If he had got up earlier this morning，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应用would have caught。
如：He remainedvery calm, otherwise he would have been found out.=He remainedvery calm, otherwise he would have been found out(if he hadn’t remainedvery calm).
高考链接
Yesterday Jane walked away from the discussion. Otherwise, she _______something she would regret later. (’96 上海)
A. had said B. said
C. might say D. might have said
评析：选D。分析题干，前一分句暗示了过去的事实，而句中副词otherwise暗示了条件，并说明了后一分句中的一个非真实情况，表示与过去事实相反。后一分句实际上省略了一个与过去事实相反的条件句If she had walked away from the discussion，故后一分句应视为省略了条件句的主句结构，与过去事实相反，应选用might have said。
4．介词或介词短语暗示的虚拟结构
在一个简单句式结构中，有时假设的条件不通过条件从句直接表达出来，而隐含在某些介词短语中。常见的介词或介词短语有without, but for等。
例1．Without electricity, human life would be quite difficult today.
如果没有电，今天人类的生活就完全不同了。
评析：该句为一简单句，句中介词短语Without electricity暗示条件，实际上相当于一个明确的条件状语从句If there were no electricity。根据句意及是句中时间状语today说明句子是对现在的假设，所以，该句应是与现在事实相反的动词虚拟形式。
如：Without his advice, I couldn’t have doneit all by myself.=If it hadn’t been for his advice, I couldn’t have doneit all by myself.
如果没有他的建议，我就不可能独自做好这一切。（句中动作与过去事实相反）
例2．But for your help, we wouldn’t have succeeded in doing this experiment.
要不是你的帮助，我们就不能成功地做好这个实验。
评析：该句与上例类似，为一简单句结构。句中介词短语But for your help暗示条件，实际上相当于一个明确的条件状语从句If it hadn’t been for your help。尽管句中没有明确的时间状语，但根据句意可知，这个句子是对过去情况的假设，故句中的谓语动词应用与过去事实相反的虚拟结构形式。
如：But for the meeting, I would have goneto your party last Saturday.=If it hadn’t been for the meeting, I would have goneto your party last Saturday.
要不是那个会议，我就参加了你上周六举行的晚会了。（句中动作与过去事实相反）
高考链接
Without the air to hold some of the sun’s heat, the earth at night______, too cold for us to live. （‘97 上海）
A.would be freezing cold
B. will be freezing coldly
C.would be frozen cold
D. can freeze coldly
评析：选A。句中介词短语Without the air to hold some of the sun’s heat暗示条件，实际上相当于一个明确的条件状语从句If there were no air to hold…。根据句意，句中时间状语today说明句子是对现在的假设，所以，该句应用与现在事实相反的动词虚拟结构形式。
5．分词短语暗示的虚拟结构
在一个简单句式结构中，有时句中假设的条件隐含在现在分词短语或过去分词短语之中。句子的谓语动词结构必须根据分词短语所假设的情况或句中的时间状语来判断和选用。
例如：Having known earlier, we would have stopped such a thing from happening again.
如果我们早点知道这件事的话，我们就会阻止它再次发生了。
评析：句中Having known earlier为现在分词短语作状语，暗示条件，实际上相当于一个条件状语从句If we had known earlier。分析句意，该句是对过去情况的假设，因此，句中的谓语动词应选用与过去事实相反的虚拟结构形式would have stopped。
如：Having been given more money, we would have been to Hongkong instead of Dalian.=If wehad been given more money, we would have been to Hongkong instead of Dalian.
如果我们有更多的钱，我们就去了香港而不是去大连了。（句中动作与过去事实相反）
Being your age, I would studyharder.=If I were your age, I would studyharder.
如果我是你这个年龄，我会更加努力学习的。（句中动作与现在事实相反）
6．上下文语境中的虚拟结构
在英语句式中，有时既没有时间状语暗示虚拟条件，也没有介词短语或分词结构来暗示条件，而其虚拟条件是由上下文表达出来。这时，我们只有通过分析上下文所给条件，才能弄清句中动词的虚拟结构。
例如： ---He would have failed in the experiment last time.
---Luckily he followed your advice.
评析：根据对话情景可知，应答句Luckily he followed your advice（很幸运他听了你的建议）实际上就是上一句讲话人暗示的条件。从应答句所给特定的动词时态（一般过去时）看，说明上一句中讲话人所说内容是对过去情况的假设，故暗示的条件句应为If he hadn’t followed your advice。由此可见，选项部分应为主句的谓语动词，与过去事实相反，应选用would have failed。
如：---I would have come to your birthday party last Sunday.
---Unfortunately, you were out on business.
= ---I would have come to your birthday party last Sunday (if I hadn’t beenout on business)
---Unfortunately, you were out on business.
---上个星期天（如果我没有出差），我就来参加了你的生日晚会了。
---很不幸，你出差了。