定義：在英語中用來修飾一個名詞、代詞的句子在語法上稱為定語從句，且定語從句放在被修飾詞的后面。定語從句分為：限制性定語從句和非限制性定語從句。
限制性定語從句：修飾的詞代表一個、一些、一類特定的人或物，這種定語從句已和被修飾詞構成了一個完整的意思，一般把限制性定語從句譯為“``````的”，定語從句不能隨便拿掉。
關系代詞一般用法一覽表：
先行詞/關系代詞
作 用 主語 賓語 介詞+賓語 定語
人 類 Who,
That Whom
That whom Whose
事 物 類 Which
that Which
that Whose Whose
用 法 說 明 不能省略 常可省略 介詞的選擇根據習慣搭配以及作用與
含義，短語動詞中的介詞不能拆開
關系副詞一般用法一覽表：
關系副詞 作用 用法 說明
When 狀語 跟在表時間的先行詞后 相當于表時間的介詞
+which的作用
Where 狀語 跟在表地點的先行詞后 相當于表地點的介詞
+which的作用
why 狀語 跟在表原因的先行詞后
（以reason為常見） 相當于表原因的介詞
+which的作用
I. 關系代詞
引導詞：which, that, who, whom, whose
具體用法：1、當先行詞為物時，關系代詞在定語從句中作主語，賓語，定語。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. 當先行詞為人時，關系代詞為that, who, whom, whose。
在定語從句中作主語時用that, who 作賓語時用 whom, who, that
作定語時用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引導的定語從句
(1) As引導的定語從句一般和such 連用：
There is no such place as you dream of in all this world.
全世界決沒有像你夢想的地方。
I like such a fine city as Beijing.
我愛北京那樣美麗的城市。
The waves were such as I never saw before.
這樣的波浪我還從未見過。
(2) 有時和the same 連用：
I have the same trouble as you (have).
我和你有同樣的困難。
This is the same bag as I lost yesterday.
這個書包和我昨天丟失的相似。
He is of about the same age as you (are of the age).
他和你年齡大約相同。
Is this the same as you showed me before?
這個和你以前給我看過的那個是一樣嗎？
（3）as引導的定語從句，也可以用在下面這類句子中：
He stopped the idea, as could be expected.
正如可以預料的，他反對這個意見。
As we all know, air is a gas.
在這種從句中，關系代詞as 可作主語，賓語或表語。
as從句中修飾全句時有“正有”的含義，而 which 從句中沒有此含義。
注意：下列四種常見錯誤
1. 在定語從句中加了多余的賓語：Some of the boys I invited them didn’t come.
2. 定語從句謂語動詞的單復數弄錯
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. 省略了定語從句中作主語的關系代詞
eg. Children eat a lot of sugar often get bad teeth.
The key opens the room is missing.
4． 定語從句中加了多余的關系副詞或介詞
eg. The house where he lives in needs repairing.