高中英语学习中，语法从句是重要的考试内容，包括名词性从句（主语、宾语和表语从句）、定语从句和状语从句。掌握从句的类型和从句与主句之间的关系是正确使用语法从句的关键。

名词性从句

名词性从句分为主语从句、宾语从句和表语从句。主语从句在句子中充当主语，如：

What he said is true. (他说的是真的。)

Whether he will come is uncertain. (他是否会来还不确定。)

宾语从句在句子中充当宾语，如：

I know that you are busy. (我知道你很忙。)

She asked me where I was going. (她问我我要去哪里。)

表语从句在句子中充当表语，如：

His problem is that he is too lazy. (他的问题是他太懒了。)

The fact is that he didn't pass the exam. (事实是他没有通过考试。)

定语从句

定语从句用来修饰名词或代词，如：

The book that I am reading is very interesting. (我正在读的那本书非常有趣。)

The girl who is singing is my friend. (唱歌的那个女孩是我的朋友。)

状语从句

状语从句用来表示时间、原因、条件、结果等，如：

When I arrived, they had already left. (当我到达时，他们已经离开了。)

Because he was sick, he didn't go to work. (因为他病了，他没有去上班。)

If it rains tomorrow, we will stay at home. (如果明天下雨，我们会呆在家里。)

He studied hard, so he passed the exam. (他努力学习，所以他通过了考试。)