定语从句的引导词辨析

关系代词that：指物，有时也指人，在从句中作主语或宾语，作主语不可省略，做宾语可省略（在非正式文体中）eg: the boy that helped me is my brother. the building that we built last year is very tall.
关系代词which:指物，不指人，比that正式，在从句中作主语或宾语，作主语不可省略，做宾语可省略（在非正式文体中）。 eg: the coat (which/ that) i put on the desk is black.
关系代词whom：指人，在从句中作宾语。 eg: everyone loves the girl whom we call Mary.（此处亦可用who/ that/省略）
关系代词whose：在从句中作定语，不可省。eg: everyone loves the girl whose watch is new.
关系副词where：在从句中作状语。eg: my wife wants to go to the city where we knew each other. ( where= in which)
what不引导定语从句，所以在定语从句中不能用what代替that。因为that是关系代词而what是连接代词，相当于the things that/ which...，也就是说what包含that. eg: do the students understand what you said? = do the students understand the words that you said?
此外，还有一些情况只能用that或which或who,可以看看这个http://baike.baidu.com/view/56536.htm