定义：在英语中用来修饰一个名词、代词的句子在语法上称为定语从句，且定语从句放在被修饰词的后面。定语从句分为：限制性定语从句和非限制性定语从句。
限制性定语从句：修饰的词代表一个、一些、一类特定的人或物，这种定语从句已和被修饰词构成了一个完整的意思，一般把限制性定语从句译为“``````的”，定语从句不能随便拿掉。
关系代词一般用法一览表：
先行词/关系代词
作 用 主语 宾语 介词+宾语 定语
人 类 Who,
That Whom
That whom Whose
事 物 类 Which
that Which
that Whose Whose
用 法 说 明 不能省略 常可省略 介词的选择根据习惯搭配以及作用与
含义，短语动词中的介词不能拆开
关系副词一般用法一览表：
关系副词 作用 用法 说明
When 状语 跟在表时间的先行词后 相当于表时间的介词
+which的作用
Where 状语 跟在表地点的先行词后 相当于表地点的介词
+which的作用
why 状语 跟在表原因的先行词后
（以reason为常见） 相当于表原因的介词
+which的作用
I. 关系代词
引导词：which, that, who, whom, whose
具体用法：1、当先行词为物时，关系代词在定语从句中作主语，宾语，定语。用which, that, whose+n, of+which
eg. (1) They planted the trees ______didn’t need much water.
(2) The meat ______we bought yesterday is not fresh.
(3) He lives in the room _______windows face the south.
(4) He lives in the room the windows ____________face the south.
2. 当先行词为人时，关系代词为that, who, whom, whose。
在定语从句中作主语时用that, who 作宾语时用 whom, who, that
作定语时用whose
eg. (1) The scientist _______visited our class yesterday is from Canada.
(2) The person _______you met yesterday is our Chinese teacher.
(3) The person _______you talked to just now is our Chinese teacher.
(4) Do you know the student ________pronunciation is the best in our class.
3.as引导的定语从句
(1) As引导的定语从句一般和such 连用：
There is no such place as you dream of in all this world.
全世界决没有像你梦想的地方。
I like such a fine city as Beijing.
我爱北京那样美丽的城市。
The waves were such as I never saw before.
这样的波浪我还从未见过。
(2) 有时和the same 连用：
I have the same trouble as you (have).
我和你有同样的困难。
This is the same bag as I lost yesterday.
这个书包和我昨天丢失的相似。
He is of about the same age as you (are of the age).
他和你年龄大约相同。
Is this the same as you showed me before?
这个和你以前给我看过的那个是一样吗？
（3）as引导的定语从句，也可以用在下面这类句子中：
He stopped the idea, as could be expected.
正如可以预料的，他反对这个意见。
As we all know, air is a gas.
在这种从句中，关系代词as 可作主语，宾语或表语。
as从句中修饰全句时有“正有”的含义，而 which 从句中没有此含义。
注意：下列四种常见错误
1. 在定语从句中加了多余的宾语：Some of the boys I invited them didn’t come.
2. 定语从句谓语动词的单复数弄错
eg. (1) Those who has finished may leave the classroom now.
(2) This is one of the rivers in China which flows northward.
(3) This is the only one of the rivers in China which flows northward.
3. 省略了定语从句中作主语的关系代词
eg. Children eat a lot of sugar often get bad teeth.
The key opens the room is missing.
4． 定语从句中加了多余的关系副词或介词
eg. The house where he lives in needs repairing.