注: 主句中的should只用于I、we，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。

l、表示与现在事实相反的假设和结果。如: If my brother were here, everything would be all right. 要是我哥哥在这儿 ，一切都没问题了。

2、表示与过去事实相反的假设和结果。如: If you had taken my advice，you wouldn't (couldn't) have failed in the exam. 如果你按照我的建议去做，你一定不会(不可能)考试不及格。

3、表示与将来事实可能相反的假设和结果。如: If it were Sunday tomorrow, I should (would，could，might) go to see my grandmother. 如果明天是星期天，我就 (可能)去看望我奶奶。If it were to snow this evening, they would not go out. 如果今晚下雪，他们将不出去了。

4、有时条件从句中的动作和主句中的动作发生的时间不一致（表示错综时间的虚拟语气），这时动词的形式要根据它所表示的时间加以调整。例如: If you had listened to the doctor, you would be all right now. 如果你当初听了医生的话，身体现在就好了。(从句动作指过去，主句动作指现在)

5、虚拟条件句可以转换成下列形式:

(l)省略连词if。在书面语中，如果虚拟条件从句中有were，had 或 should，可以把if省略，把这几个词放到主语之前，构成主谓倒装。例如: Should he come (If he should come), tell him to ring me up. 他要是来了，让他给我打个电话。Were I you (If I were you), I would not do it. 我要是你，就不做这事。

(2)用介词短语代替条件状语从句。如: Without air (If there were not air), there would be no living things. 如果没有空气的话，就不会有生物了。But for your help (If it hadn't been for your help) I couldn't have done it. 要是没有你的帮助，我就不可能完成这件事。 I was busy that day. Otherwise I would have gone there with them. (If I hadn't been busy that day, I would have gone there with them.) 我那天很忙，否则，我就和他们一起去那儿了。(如果我那天不忙的话，我就......)；I would have finished the work, but I have been ill. (If I hadn't been ill, I would have finished the work.) 我本来该完成这项工作的，但我生病了。(如果我没生病的话，我就会完成......)

6、省去条件从句或主句：表示虚拟语气的主句或从句有时可以省略，但其含义仍可以推知。

(1)省去条件从句。如: You could have washed your clothes yourself. 你本可以自已洗衣服的。省去了"If you had wanted to")(事实是:你自己没洗衣服，因为你不想洗。)