在复合句中，修饰一个名词或代词的从句叫做定语从句；被定语从句所修饰的词叫先行词；定语从句必须放在先行词之后；引导定语从句的词叫关联词。如：
〔关联词〕
常见的关联词有关系代词that，which，who，whom，whose和关系副词where，when，why等。
〔种类〕
1．由that引导的定语从句
一般情况下that可指人或物，可以代替who，whom和which，在从句中作主语或谓语动词的宾语，但不能作介词的宾语。如：
The comrade that（who）is speaking at the meeting is an advanced worker．正在会上讲话的那个同志是个先进工人。
Is this the doctor that（whom）you talked about yesterday？这就是你们昨天谈论的那位医生吗？
The letter that（which）I received yesterday was from my father．昨天我收到的信是我父亲寄来的。
2．由who，whom和whose引导的定语从句
who在从句中作主语，whom在从句中作宾语，whose在从句中作定语。如：
This is the thief who stole my bike．这就是偷我自行车的那个贼。
He is the boy whom you wanted to find．他就是你想要找的那个男孩。
The girl whose mother is a doctor is waiting for you outside．妈妈是个医生的那个女孩在外面等你。
3．由which引导的定语从句
which指物，在从句中作主语或谓语动词的宾语。如：
The book which Mum bought me last week is called“Red Star Over China”．妈妈上周买给我的那本书名叫《西行漫记》。
The house which Lu Xun once lived in is now the Lu Xun Museum．鲁迅曾经住过的那所房子现在是鲁迅博物馆。
4．由where，when和why引导的定语从句
where在从句中作地点状语，when在从句中作时间状语，why在从句中作原因状语。如：
I will never forget the day when I joined the League．我永远忘不了我入团的那一天。
He will go back to the school where he studied next week．下周他要回到他曾经学习过的学校。
I don't know the reason why he quarreled with Zhang Lin．我不知道他同张琳吵架的原因。
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