定语从句是英语语法中一个重要的部分，它是一种修饰或限制名词或代词的句子，用于加强表达力和详细阐述名词或代词的特征。定语从句的构成方法非常灵活，可以使用关系代词或关系副词。
一：关系代词
关系代词是定语从句中最常用的连接词，常见的关系代词有who、whom、whose、which和that。这些关系代词的使用方式不同，who和whom用于指人，whose表示所属关系，which用于指物，that既可以指人也可以指物。例如，“The girl who is standing at the door is my sister.” “The book which I borrowed from the library is very interesting.”
二：关系副词
关系副词也可以用于连接定语从句，它们与关系代词有些类似，但与之前所学的副词类别不同。常见的关系副词有where、when和why。where用于指地点，when用于指时间，why用于指原因。例如，“The restaurant where we had dinner last night was very expensive.”
三：练习题
以下为定语从句练习题：
1. This is the man ___ helped me.
A. which B. who C. whom D. that
2. The car ____ we bought yesterday is very fast.
A. which B. that C. whose D. whom
3. The reason ____ he came late was because of traffic.
A. which B. why C. when D. where
定语从句是一项基本的英语语法知识，在口语和写作中都会经常用到。通过不断的练习和运用，我们能够更加自如地表达自己的意思，使语言表达更加准确和流畅。
