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(be的过去式用were) |
主语+ would/should/could/might+原形动词 |
例如：①If I feared death, I wouldn't be a communist.怕死就不做共产党员。
②If there were no air, we couldn't live.如果没有空气，我们就不能生活。
2.与过去事实相反的虚拟条件句
| 条件状语从句 |
主句 |
| If + 主语 + 动词过去完成式 |
主语 + would/should/could/might+have + 过去分词 |
例如：①If 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。
②If 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。
3.与将来事实相反的虚拟条件句
| 条件状语从句 |
主句 |
| If + 主语+ should/were to +原形动词 |
主语+would/should/could/might+原形动词 |
例如：①If he were to come tomorrow, things would be easier.假如他明天会来，事情就容易办了。
②If 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。
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：①If 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）
②If 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）。
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？
