英语语法中的三种主要从句包括名词性从句、形容词性从句和副词性从句。名词性从句在句子中充当名词，形容词性从句用来修饰名词或代词，而副词性从句用来修饰动词、形容词或副词。这三种从句在语法结构、用法和作用上都有所不同，学习英语的同学们应认真区分和掌握它们的用法。
名词性从句
名词性从句就是在句子中充当名词的从句，通常包括主语从句、宾语从句和表语从句。其中，主语从句作为句子的主语，宾语从句作为动词的宾语，表语从句则作为主语或宾语的补语。名词性从句通常以连词that, whether, if等开头，例如：
- 主语从句：That he has passed the exam is really good news.
- 宾语从句：I don't know whether he will come to the party.
- 表语从句：Her problem is that she can't speak English fluently.
形容词性从句
形容词性从句用来修饰一个名词或代词，起到形容词的作用。它一般以关系代词who, whom, whose, that, which等开头，例如：
- The book, which was written by a famous author, is very popular.
- The girl who is sitting next to me is my best friend.
副词性从句
副词性从句用来修饰一个动词、一个形容词或一个副词，起到副词的作用。它一般以连词because, if, when, although等开头，例如：
- I will stay at home because it is raining outside.
- If you study hard, you will pass the exam.
