关系代词有who，whose，whom，that，which等，用来引导定语从句。它们在定语从句中可用作主语、宾语、定语或表语;另一方面它们又代表主句中为定语从句所修饰的那个名词或代词(这样的名词和代词叫做先行词)。如：

The man who is talking with Mr. Brown is an eye Specialist和布朗先生谈话的那个人是个眼科大夫。(关系代词who在从句中用作土语，它的先行词是man)

He is the comrade whom you have been looking for他就是你要找的那位同志。(关系代词whom在从句中作宾语，它的先行词是comrade，whom在口语中一般可省去)

The old man whose son is in the navy used to be carpenter.儿子在海军的那位老人过去是个木匠。

(关系代词whose在从句中用作定语，先行词为man)

The film which I saw last night is about a young teacher.我昨晚看的那部电影说的是一个年轻教师的事。(关系代词which在从句中用作宾语，先行词为film, which在口语中可省略)

This is the plane that will fly to Tokyo in the afternoon.这是下午要飞往东京的飞机。(关系代词that在从句中用作主语，先行词为plane)