定语从句指的是一个句子跟在一个名词或代词（先行词）后进行补充、修饰、限定，从而使原句内容更完整。在整个句子中，定语从句充当定语的成分。被定语从句修饰的词叫先行词，定语从句不同于单词作定语的情况，它通常只能放在被修饰的词（即先行词）之后。 定语从句由关系词（关系代词、关系副词）引导，关系代词、关系副词位于定语从句句首。 由关系代词引导的定语从句可以分为表示人的或者表示物的。关系代词所代替的先行词是人或物的名词或代词，并且在句中充当主语、宾语、表语等成分。

表示人的关系代词有：who, whom, that, whose.

如： The boy who are playing the piano is my brother. 正在弹琴的那个男孩是我弟弟。 The girl whom I sent the present is beautiful. 我送她礼物的那个女孩很漂亮。 She is the lady that wants to see you. 她就是想见你的那位女士。 I know a girl whose father is a lawyer. 我认识一个父亲是律师的女孩。 表示物的：which、whose、that. 如: I don’t like the city which he lives in. 我不喜欢他住的那座城市。 That notebook whose cover is black is mine. 那本封面是黑色的笔记本是我的。 She gives the best speech that I have listened to. 她做了一场我听过最棒的演讲。 注意，关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。

一些情况只能用that做关系代词： （1）不定代词anything, nothing, everything, all, much, few, any, little为先行词时，例如： I don’t have anything that you want. （2）先行词为the only, the very, the just所修饰: He is the only man that I can trust. （3）先行词被序数词、数词、形容词或最高级修饰时: She is the most beautiful lady that I have ever seen.

（4）先行词既有人又有物时，如： He talked about the teachers and school that he visited.

（5）当主句是以who或which开始的特殊疑问句时，为避免重复用that: Who is the person that you were talking to?

（6）关系代词在从句中作表语时: He is not the man that he used to be. 由关系副词引导的定语从句，关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。常用的关系副词有：when、where、why、that等。

例如： I still remember the days when we was in Beijing. 我还记得我们在北京的那些天。 This is the house where I have lived for 3 years. 这是我住了3年的房子。 I can’t find the place that we lived 10 years ago. 我找不到我们10年前住过的地方。 I don’t know the reason why he was late.我不知道他迟到的原因。

另外，定语从句还可以分为限制性和非限制性定语从句。限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确；非限制性定语从句不直接修饰先行词，只为先行词提供一些补充的信息，是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，将非限定性定语从句放在句子中间，其前后都需要用逗号隔开。

限制性定语从句： She has found the necklace that she lost two weeks ago. Do you know the professor who is speaking at the meeting?

非限制性定语从句： She is an artist，which I am not. She is always careless，which we should not be.