关系代词在从句中充当主语、宾语、定语（whose, which）。
表示人时用who（主格/宾格）, whom（宾格，也就是动词或介词的宾语）, that, whose（所有格）；表示物时用which, that, whose。
（注意that和whose既可表人也可表物），另外which可以指代前面整个句子；注意，介词后宾语表人用whom，表物用which，若是表所有格，用whose。
特别注意：what不能引导定语从句，只能引导名词性从句。
我们来看以下几个例句：
The girl who/that is standing at the gate is an English teacher.(who或that作从句的主语, 表人)
That’s not Tom who/that is sitting next to John. (who或that作从句的主语, 表人)
=That’s not Tom. Tom is sitting next to John.
The girl whom/who/that Peter is talking to is a teacher. (whom作从句宾语,表人)
The girl to whom Peter is talking is a teacher. (介词可以前置)
The girl to who/that× Peter is talking to is a teacher.×（当介词前置时，表人只能用whom）
(人) The girl (whose mother is a nurse) is a teacher.(whose与它所修饰的名词一起构成一个名词短语在从句中担当成分)
(物) The book (whose cover is missing) is very funny.(不可分割的一部分, 指物时可用of which来代替)
= The book, the cover of which is missing, is very funny.
The room (whose door faces south) is very bright. (物质所属)
= The room, the door of which faces south, is very bright.
The book (which/that belongs to me) is missing.（which/that作从句主语，表物）
I don’t like the novel (that/which you are reading).(that/which作从句动词的宾语，表物)