什么时候用that 什么时候用what

what
1.引导名词性从句：I don't understand what he is talking about.
(此句中what在名词性从句里做宾语）
It is difficult to decide what should be done first.
(此句中what在名词性从句里做主语）
2.引导感叹句：I can't believe what a beautiful girl she is.
that
that在定语从句中做主语、表语或宾语,作宾语时可省略.
必须用that的情况：
先行词有人又有物
先行词有形容词最高级修饰
先行词是不定代词
先行词有不定代词修饰
以who,which开头的问句
先行词有the very,the same ,the last等词修饰
先行词有序数词,数词修饰时.
关系词在定语从句中做表语时
不能用that的情况：
介词+关系词
非限定性定语从句
举例：
Is this the book that you borrowed in the library?
这是你在图书馆借的那本书吗?
Who that break the window should be punished．
谁打碎了窗户都要受到惩罚．
All that is needed is a supply of oil.
所需的只是供油问题.
Finally,the thief handed everything that he had stolen to the police.
那贼最终把偷的全部东西交给了警察.

that
与
what
都可引导主语从句，
that
在从句中不作句子成分，但不能省略。
what
在从句中可作主语、
宾语、表语等，
不能省略。如：

What he said at the meeting was very important．他在会上所说的非常重要。
宾语）What surp...

He did not comment on what I said.我的话就是指我说的内容，那就是有实际含义的，什么之类。而that没有包含说话内容的意思。若是有that 那么就缺少了said的宾语，结构不完整。
故选用what，就像what to say/do之类的。