条件句有两类，一类是真实条件句，一类是虚拟条件句。如果假设的情况是有可能发生的，就是真实条件句。在这种真实条件句中的谓语用陈述语气。e.g. If it doesn't rain tomorrow, we will go to the park.

　　如果假设的情况是过去或现在都不存在的，或将来不大可能发生的，则是虚拟条件句。

如: If he had seen you yesterday, he would have asked you about it.

如果他昨天见到你，他会问你这件事的。(事实上他昨天没见到你，因此也未能问你这件事。)

在含有虚拟条件句的复合句中，主句和从句的谓语都要用虚拟语气。现将虚拟条件从句和主句的动词形式列表如下:

　　　　　　　　　虚拟语气在条件从句中的用法

从句 主句 与现在事实相反 If+主语+动词过去式 (be用were) S+would / could / might / should +do 与过去事实相反 If+主语+过去完成式(had +done) S+would / could / might / should +have +done 与将来事实相反 1）If+主语+动词过去式 (be用were)

2）If+主语+should + do

3）If+主语+ were to do S+would / could / might / should +do 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。

If I had the chance, I might want to be a translator.

If she knew as much about music as you do, my daughter would play the piano much better.

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。