虚拟语气是一种特殊的动词形式，表示所说的话不是一个事实，而只是一种假设，愿望，建议，怀疑，猜测或不大可能实现的空想。
I.虚拟语气用在条件状语从句中
虚拟语气用在条件状语从句中，通常从句由连词if 引导。
1.与现在事实相反的虚拟条件句
| 条件状语从句 | 主句 |
|---|---|
| If +主语 +动词过去式 | 主语+ would, should, could, might+原形动词 (be的过去式用were) |
例如：
If I feared death, I wouldn't be a communist.
（刘胡兰）怕死就不做共产党员。
If there were no air, we couldn't live.
如果没有空气，我们就不能生活。
Example:
I would ask George to lend us the money if I ______ him.
A. had known B. knew C. have known D. know
I _____ it to him if I thought he would understand.
A. shall explain B. will explain C. would explain D. explain
2.与过去事实相反的虚拟条件句
构成
| 条件状语从句 | 主句 |
|---|---|
| If + 主语 + 动词过去完成式 | 主语 + would, should, could, might+have + 过去分词 |
例如：
If you had come a few minutes earlier, you would have met him.
如果你早来几分钟，你就会碰上他。
If there had been no air in the tube, the result of the experiment would have been more accurate.
如果试管中没有空气，实验结果就会更精确。
Example:
If you hadn't gone with Tom to the party last night, _______.
A. you would meet John already
B. you won't have missed John
C. you will have met John
D. you would have met John
3.与将来事实相反的虚拟条件句
构成
| 条件状语从句 | 主句 |
|---|---|
| If + 主语+ should +原形动词 | 主语+would,should,could,might或were+不定式+原形动词 |
例如：
If he were to come tomorrow, things would be easier.
假如他明天会来，事情就容易办了。
If the sand should be broken up, it would give out much energy.
如果沙子能被分解，它会释放出大量的能量。
注：在表示与将来事实相反的虚拟条件句中，从句中的should有时可以省略。如：
If he make the design, he might do it in some other way.
要是他来搞这个设计，他可能用另外的方法设计。
Example:
If the sea ____ to rise 50 feet, India would become an island.
A. will B. is C. was D. were
If you ____ to see Mary, what would you tell her?
A. are B. will be going C. must D. were
We might still catch the train if we ___.
A. make hurry B. haste
C. make haste D. hastily
4.错综时间的虚拟语气
在一些含有虚拟语气的句子中，如果主句与从句的谓语表示不同的时间，虚拟语气可有不同形式，即主句与从句各自按自己的时间构成虚拟形式。例如：
If I had met him before, I could recognize him.要是我以前遇见过他，我就能认出他了。
（从句表示与过去事实相反，主句表示与现在或将来事实相反）
If we were driving at a speed of 100 Li an hour, we would have arrived there 2 hours ago. 如果我们的行车速度每小时100里，两小时以前我们就会到达那里了。
（从句与现在事实相反，主句与过去事实相反）
Example:
If you _____ that late movie last night, you wouldn't be sleepy.
A. haven't watched
B. didn't watch
C. hadn't watched
D. wouldn't have watched
5.省略连词if
在条件状语从句中，如果有were, should, had，可以把连词if 省去，把 were, should, had 放在句首，构成条件意义。例如：
Should it rain tomorrow, what would you do? 万一明天下雨，你怎么办？
Were there no friction, we could not walk. 要是没有摩擦力，我们就不能行走。
Had we known about the new method, we should have applied it earlier.
如果我们过去知道这种新方法，我们早就应用了。
Example:
Had Alice been more hardworking, she _______.
A. had not failed B. would not fail
C. could not be failed D. would not have failed
_______ today, he would get there by Friday.
A. Would he leave B. Was he leaving
C. Were he to leave D. If he leaves
______ difficulties, we would be successful.
A. Should we overcome B. Would we overcome
C. Might we overcome D. Could we overcome
6.虚拟结构中省略主句或从句
虚拟结构中有时可省去主句或从句，并不影响意义的完整。
(1)省去条件从句
That would be fine. 那太好了。（省去了if you would come 或类似条件）
I wouldn't pass up the chance to visit China. 我不会放弃访问中国的机会。
We might have produced more coal. 我们可能生产出更多煤炭。
注：省略条件句，在试题中很少见到。
(2)省去主句，常用以表示愿望。从句谓语多用过去时或过去完成时。
If only she were here. 假如她在这儿该多好啊！
If only he lived! 假如他活着该多好啊！
If only I had known it before. 如果我早先知道这件事，那就好了。
Example:
If only it _____! but it is too late.
A. is avoided B. has avoided
C. could be avoided D. can be avoided
If only everything _____ out as we wanted it to in life!
A. is working B. worked
C. works D. has worked
