1.定语从句的关系代词在从句中作主语,宾语和表语.
2.定语从句中的关系代词有which,that,who,whom,whose和as.
3.what本身不能引导定语从句.
4.不过当先行词是all的时候,定语从句通常用that引导,如果all是主句主语,那么all that两个单词可以用what代替,不过what引导的叫主语从句；如果all在主语中作宾语,刚换成what刚引导的叫宾语从句.
5.但是that和what是不可能同时引导定语从句的.
如：All that he taught me was very useful.= What he taught me was very useful.前句中的that引导定语从句,all为先行词,而后句中的what引导的叫主语从句,what在主语从句中作taught的直接宾语.
不知道我解释清楚没有,希望对你有所帮助.