引导定语从句的词有关系代词which, that, who(宾格whom,所有格whose)和关系副词where, when, why。它们放在先行词和定语从句之间，起联系作用，同时又作定语从句的一个成分。
一．关系代词的用法：
　　1.who只可指人，在从句中作主语；that也可指人，在句中作主语或宾语。作主语时，who和that可以互换，作宾语时，that可省略。
　　The students who /that do not study hard will not pass the exam.学习不努力的学生考试将不会及格。(作主语)
　　The boy(that)we saw yesterday is John&s brother.我们昨天看到的那个男孩是约翰的兄弟。(作宾语)
　　2.which和that都可指物，在从句中作主语或宾语。作宾语时，可省略。
　　The cat which /that is under the chair is called Mimi.椅子下的那只猫叫******。(作主语)
　　The story(which /that)he told us is very interesting.他给我们讲的那个故事很有趣。(作宾语)
　　3.whom是who的宾格，只可指人，在句中作宾语，可省略。
　　The comrade(whom)you want to see has come.你要见的那个同志已经来了。(作宾语)
　　4.whose是who的所有格，指人，也可指物，在句中作定语，不能省略。
　　He is the person whose illness is of long standing.他是一个长期患病的人。(作定语)
二．关系代词who和that在用法上的异同：
　　在通常情况下，指人时who与that在定语从句中可以互换。但在下列情况下只能用who，而不能用that。
　　1.当先行词为one, ones, 或anyone时。
　　The girl is one who gave me much help.那位女孩是一位给了我很大帮助的人。
　　2.当先行词为those时。
　　Those who work hard will pass the exam.那些学习用功的人考试会及格。
　　3.在以there be开头的句子中。
　　There is a policeman who wants to see you.有一位警察要见你。
　　There lived a young man who was an artist on the second floor.二楼住着一位青年画家。
三．关系代词that，which和who的用法区别：
　　在下面几种情况要用that,不能用which或who.
　　1.当先行词为all,everything,nothing,something,anything,little,few,any,no等不定代词时。
　　Is there anything that I can do for you?有什么事情我能帮忙吗？
　　All that he dreams of comes true.他的一切梦想都实现了。
　　2.当先行词被all,every,no,some,any,little,much,the only,the last,the same,the very等词修饰时。
　　He is the very /only man that I wanted to see.他正是我想要见的那个人。
　　This is the same bike that I lost last month.这就是我上月丢的那辆自行车。
　　3.当先行词被序数词或形容词最高级修饰时。
　　This is one of the most exciting football games that I have ever seen.这是我所见过的足球赛中最激烈的一场。
　　The first English novel that I read was A Tale of Two Cities.我读的第一本英文小说是《双城记》。
　　4.当先行词既有人又有物时。
　　They talked for an hour of things and persons that they remembered in the school.他们谈论了一小时他们记得的学校里的人和事。
　　5.当主句是以who或which开头的特殊疑问句时。
　　Who is the boy that is singing over there?在那边唱歌的那个男孩是谁？
Which is the true story that he told us?他给我们讲的哪一个故事是真实的？