夜凈天明

（1）條件狀語從句

1.，用if引導條件狀語從句，從句為現在時，通常譯作 “如果。。。”
    If you get up early, you will catch up with the train

2. if 引導的條件狀語從句， 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）與現在事實相反的虛擬語氣

1. If 從句是一種虛擬的條件或假設，和現在事實相反。主從句時態具體如下：
    從句： 動詞過去式（be用were）
    主句： would/could/should/might + 動詞原形

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

（3）與現在事實相反的虛擬語氣

1. if 從句是一種虛擬的條件或假設，和過去事實相反的
    主從句的時態具體如下：、
    從句： had + 動詞過去分詞
    主句： would / could / should / might + have + 動詞過去分詞

    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