定语从句which 和what 后面所接的东西有什么区别?例子：I have decided to

定语从句：指人时常用which 和 who 而 what 则常用于指物

首先你要区分从句的概念,定语从句不是名词性从句.
名词性从句分为：主语从句,宾语从句,同位语从句和表语从句.

what可以引导名词性从句,在从句中可做主语、宾语、表语.

which是关系代词,引导定语从句,在从句中可充当主语、宾语、表语.引导定语从句的词分为关系代词（which,that,who,whom,whose,as）关系副词（when,where,why）,注意：没有what.

简单点记吧：定语从句放在表示“事物”的名词后面,且关系代词在从句中充当句子成分,用which.如果不是定语从句,就考虑用what.
如：
1.This is the book which I talked about yeasterday.
(which引导定语从句放在名词book后面起修饰作用,which在从句中充当talk about的宾语)
2.This book is what I talked about yesterday.(what引导名词性从句,放在is之后,what 在表语从句（名词性从句）充当talked about的宾语.)