定语从句(Attributive Clause)是指在复合句中用作定语的从句,又称为关系从句(Relative Clause)。但与一般定语不同的是，定语从句一般都位于它所修饰的名词或代词之后。被定语从句所修饰的名词或代词称为先行词。引导定语从句的关系词有关系代词who, whom, whose, that, which等和关系副词when, where, why等。另外，as,but 等也可以作为关系代词来引导定语从句。根据定语从句与先行词之间关系的紧密程度，定语从句可分为限制性定语从句和非限制性定语从句两种。
关系代词who, whom（代人）, which（代物）, that, whose（代人或代物）在定语从句中作主语，宾语，定语等。关系代词在定语从句中作动词宾语时一般可以省略。关系代词的人称和数要和先行词一致，它的格取决于它在从句中充当的成分。
The man who did the robbery has been caught. (who 在从句中作主语)
Then I telephoned the doctor (whom)Charles had recommended. （whom 在从句中作宾语，在口语中常用who代替，也可省略）
It is a question that/which needsvery careful consideration. （that/which在从句中作主语）
Edison is a great inventor whosefame is worldwide. （whose在从句中作定语，指人）
The house whose windows are broken is unoccupied.（whose在从句中作定语，指物）
He is not the man that he was.（that指人，在从句中作表语）
He that would eat the fruit must climb the tree.（谚语，that指人，在从句中作主语）
You can take any seat that is free. （that指物，在从句中作主语）
Who is the man that is waiting for you under the tree? （在疑问句中，当疑问词为who时，关系代词则要用that指人）
l 提示：当先行词为all, everything, anything等不定代词，或先行词之前有all, no,
any, little,only, very等限定词、序数词或形容词高级等修饰时，其后的关系代词通常要用that。
All (that) you have to do is to press the button. （此句中关系词必须用that，也可以省略）
That’s the very textbook that weused last term.
He is the bravest man that everlived. （此句中that不能换成which）
关系副词when, where, why等在定语从句中作状语，分别表示时间，地点和原因；它们的先行词分别是表示时间，地点和原因的名词。
There are moments when I forget allabout it.
This is the factory where my fatherworks.
The reason why he came is not very convincing.
As用作关系代词引导定语从句时，可在从句中作主语，表语或宾语，先行词前一般有the same, such等词。But也可以用作关系代词引导定语从句，用于否定词语之后，在语义上相当于that/who…not。
This is the same bag as I lostyesterday. （as指物，在从句中作宾语）
Such people as you describe are rare nowadays. （as指人，在从句中作宾语）
Such people as knew Tom thought he was a talented youngman. （as指人，在从句中作主语）
There are very few but (= who don’t) admire his talents.
There is no man but (=who doesn’t)feels pity for starving children.
I have a brother who works in achemical works. 我有一个在化工厂工作的哥哥。（意即：我还有别的哥哥，但他们不在化工厂工作。who引导限制性定语从句。）
I have a brother, who works in achemical works. 我有一个哥哥，他在化工厂工作。（意即：我只有一个哥哥，非限制性定语从句只是补充说明他的情况。）
My new car, which I paid several thousand pounds for, is notrunning well.
A boy took us into the physics classroom,where Jack was sitting on the lecture table.
The games in which the young mancompeted were difficult.
I’ll never forget the day on which Ivisited the Great Wall for the first time.
The documents for which we weresearching have been recovered.
They admired the way (in which) shedealt with the crisis.
The athlete to whom you talked is afamous runner.
He is a man on whom you can safelydepend.
I called him by the wrong name, forwhich I apologized.
There are about 100 foreign students intheir university, many of whom are from Asia.
She often changes her mind, whichmakes me angry.
He has to work on Sundays, which hedoesn’t like.
He opposed the plan, as could beexpected.
As is well known, the earth is one of the nine planetsin the Solar System.