What只能在主语从句里做主语？

专业点、
我是分不清宾语/主语从句 和定语从句、

可以用在包括主语从句的各类名词从句。
 
1、what 称作关系代词型连接代词，相当于“先行名词 + 关系代词”，意思是“所…的”。如：
 

what he said 他所说的（话）【= something that he said】

what we will do 我们所要做的（事情）【= things which we will do】

what he has read 他所读过的（书/文章）【= a book或 an aricle he has read】
 
2、what 可以用来引导名词性从句，包括宾语从句、主语从句、表语从句或同位语从句。例如：
 

I don’t understand what you have said. 我不明白你所说的。（宾语从句）【= I don’t understand the word that you have said.】

What you told me just now is not true. 你刚才所告诉我的不是真的。（主语从句）【= The story which you told me just now is not true.】

This is what he sent me yesterday. 这就是他昨天所寄给我的。（表语从句）【This is the parcel/letter which he sent me yesterday】

The fact what the reporter published in the newspaper yesterday has been taken air. 记者在昨天报上所披露消息的真相已经传开了。（同位语从句）【The fact, the news that the reporter published in the newspaper yesterday, has been taken air.】

3、出现在宾语、主语、表语或同位语位置的从句称作宾语从句、主语从句、表语从句或同位语从句。跟在先行词（名词或代词）后，用作该先行词后置定语的句子称作定语从句；what 从句不能用作定语从句（你可以通过我所举例子后方括号中的改写认识什么是定语从句）

在引导主语从句时，what可以做主语，宾语，表语等。 What he said at the meeting astonished everybody present. what引导主语从句在从句中作宾语 What is most important in life isn't money. what引导主语从句，并在从句中作主语

what在高中阶段，需了解几个含义： 1what= the thing (that...) you can take what you like.(=you can take the thing that you like.) 2.what=the person.(that....) mary isn;t what she used to.(=mary isn't the person that she used to. 3.(in/at) the place (that....)你的句子等于： a modern city has been set up in the place that was a wasteland ten years ago. 第二问题想必你理解了吧。 第三太多了 (what he said/bought/wanted/found/did 主语) surprised us greatly. 第四问题 宾语从句除了what's wrong with.../ what's the matter with/ what's the trouble with..不改变语序外，其余均要改成陈述句式。