定语从句章节给出了如下两例句：

All (that) he said is true.

Everything (that) he said is true.

原来平时常见的这两句话都是省略过 that 的。那么随便造个句子：

What you've done is wrong.(what已经具有先行词+关系代词的语法功能,因此what前面不可以再有先行词.)

=The thing that you have done is wrong.

这句话 what 后面应该没有省略that了吧，但是感觉和上面两个例句结构是一样的呢？区别是在哪里呢？感觉问得不很清楚，但觉得三句话结构是相同的）

以上句子结构核心关键为：先行词+关系代词that/who=具有双重身份/含义连词what

(1)all that引导定语从句=what引导主语从句并在主句充当某一语法成分。

All that should be said has been said.（that作主语，不可以省略）

=What should be said has been said. （what should be said作主句的主语成分，what同时又作从句的主语成分）

(2)all that+谓语+其它，如果关系代词在定语从句作宾语成分，可以省略，不写出来,关系代词that在定语从句作主语，不可以省略，必须写出来。

All （that） he said is useless.

=All he said is useless.

=What he said is useless.