怎么判断英语定语从句关系词在句中是什么语

这很简单.句子陈述的对象就是主语；动词的对象（或内容）就是宾语；凡是修饰名词的都是定语.如：
This is the book （that/which) I want to buy.这是我想要买的书.
that/which是买的对象,作宾语.关系词如果在从句中作宾语,可以省略.
I saw a man who/that was wearing a red coat.我看见一个穿红上衣的男人.
who/that在从句中作主语.
I saw a man whose coat was red.我看见一个穿红上衣的男人.
whose修饰coat,是定语.
作主语、宾语或定语的关系词叫关系代词.
如果关系词在从句中表示的是时间、地点、原因等,就是状语.如：
Do you remember the day when we played together?你记得我们一起玩的日子吗?
when在从句中表示时间,是时间状语.
The reason why he was late is that he didn't get up early.他迟到的原因是他没有早起.
why在从句中作原因状语.
作状语的关系词叫关系副词.