1. 定義：用來修飾某個名詞、代詞或整個句子的從句就叫做定語從句。如：

The man you talked about is our headmaster. 你們剛才所談論的那個人是我們的校長

This is the bike my father gave me as my birthday present. 這是我父親送給我作為生日禮物的單車。

The tree whose leaves are red was planted last year. 那棵葉子紅色的樹是去年栽的

2. 先行詞：定語從句所修飾的詞就叫做先行詞，它一般是由名詞、代詞或句子充當。如：

The boy who is singing at the stage is only nine years old. 在舞臺上唱歌的那個男孩今年才九歲。(現行詞是名詞)

Anyone that is willing to help others is welcome to our group. 任何愿意幫助他人的人都歡迎來參加我們的組織。(現行詞是代詞)

His mother is out of danger, which excites their family very much. 他母親脫離了危險，這件事情使他們家興奮不已。(現行詞是句子)

3. 關系詞：引導定語從句的詞就叫做關系詞。根據關系詞的用法不同，關系詞又分關系代詞和關系副詞，其中關系代詞有that, which, who, whom, whose, as, but等，關系副詞有where, when, why等。關系詞既起連接先行詞與定語從句的作用，又做從句中的某個句子成分。如：

Lee is one of the students who are working very hard. 李是一個學習非常努力的學生。(引導詞who)

There is nobody but knows you here. 這兒沒有不認識你的人。(引導詞but)

He led us to a place where we have never been before. 他把我們帶到了我們以前從未去過的地方。 (引導詞where)