假设

（1）条件状语从句

1.，用if引导条件状语从句，从句为现在时，通常译作 “如果。。。”
    If you get up early, you will catch up with the train

2. if 引导的条件状语从句， 可以放在句首，也可以放在句尾
    If you ask him, he will help you
    She will be upset if you fail the exam

（2）与现在事实相反的虚拟语气

1. If 从句是一种虚拟的条件或假设，和现在事实相反。主从句时态具体如下：
    从句： 动词过去式（be用were）
    主句： would/could/should/might + 动词原形

    If i were you, I would join them
    She would come with you if you invited her

（3）与现在事实相反的虚拟语气

1. if 从句是一种虚拟的条件或假设，和过去事实相反的
    主从句的时态具体如下：、
    从句： had + 动词过去分词
    主句： would / could / should / might + have + 动词过去分词

    If i had got there earlier i should have met her
    If he had taken my advice, he would not have made such a mistake