定语从句是英语中常用的从句结构之一，用来修饰名词或代词，起到限定或说明的作用。掌握定语从句的语法规则和用法，对于提高英语表达的准确性和语言流利度非常重要。
定语从句的引导词包括关系代词和关系副词。关系代词有：who, whom, whose, which, that；关系副词有：where, when, why。引导词在从句中充当某个成分，起到连接主句和从句的作用。
下面是一些例句来说明定语从句的用法：
- The book that you borrowed from the library is due tomorrow.
- The girl who won the singing competition is my sister.
- I visited the city where I was born last summer.
- Do you remember the day when we went to the zoo?
- Can you tell me the reason why he left?