Question

什么叫主语从句，宾语从句

Answer 1

从句有三大类：名词从句，状语从句和定语从句，从句的组成与一般直陈句基本一样，但不能独立存在，要在句首加一个引导词以粘附在一个能独立存在的句子上，这样的句子称为主句。
名词从句就是从句像名词一样附在主句上，因为从句起名词作用，它可以在主句中作主语、谓语动词宾语、表语以及介词宾语，还有同位语：（下面是我从一本旧语法中摘录的有关内容）
Classification
1)Subject to Verb

That Chinese have had victory in the liberation war is a fact.
What he will declare is unknown to me.
2)Object to Verb
I do not know where he will remove.
I try to judge who will be slected.
3)Object to Preposition

Your success depends upon whether you have such determination.
He speaks in accordance with how he was instructed.
4)Complementt to Verb
This is why they oppose you.
It seemed that it had rained.
5)In apposition with a Noun

The saying that the time is valuable is true.
What is the reason that he refuses to come here?
现在简单回答你的问题：当名词从句在主句中作主语时，就称为主语从句，在主句中作
宾语时，就称为宾语从句，………
句法 是英语语法中的一重点内容，内容很多，要花大力气去学，我只能说这么多，供你参考。

Answer 2

在句子中担当主语的从句就叫做主语从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。