定语可以由形容词、代词、数词、名词、分词、不定式、介词短语等来担任，修饰名词。(以分词、介词短语为例）
例： The girl behind the tree is Kate.
The man driving too fast was drunk.
定语从句
在英语中，修饰名词或代词的句子叫定语从句。
例：The girl who is behind the tree is Kate.
The man who was driving too fast was drunk.
关系代词如何引导定语从句
普通代词与关系代词的区别
I have a sister.She works in Shanghai.
代替 sister
普通代词：只起代替的作用（如上例she)
I have a sister who/that works in Shanghai.
关系代词： （如上例who/which）