指示代词的问题

what = the thing/things that，因此what可以用于定语从句，但不需要先行词，换句话说，what其实引导的是名词从句。比如：
This is what I want. = This is the thing that I want.
这就是我想要的。
Do you know what they're doing? = Do you know the thing they're doing?
你知道他们在做什么吗？
What caused the accident were stones. = The things that caused the accident were stones.
一些石头导致了这起事故。

what在句子中是连接代词，在此句中引导表语从句，在从句中作want的宾语。