定语从句中先行词what与that的区别

更正两个概念,先行词是指被定语从句修饰的名词或代词,还有what不用来引导定语从句.
that在定语从句中的用法如下：
1.This is the book that was witten by Jim.that在从句中作主语
2.This is the book that I gave him.that在从句中作gave的直接宾语
3.He is not the man that he used to be.that在从句中作表语