当使用定语从句时，关系词（也称为关系代词或关系副词）用于引导从句，并将其与主句中的名词或代词关联起来。

以下是一些常见的关系词及其在定语从句中的用法和功能：
逗号。

以下是一些示例：
The book that is on the table is mine. (限定性定语从句，关系代词)
John, who is my friend, likes to play basketball. (非限定性定语从句，关系代词)
I visited the city where I was born. (限定性定语从句，关系副词)
The reason why he quit his job is still unknown. (限定性定语从句，关系副词)
The house, which was built in 1902, is now a museum. (非限定性定语从句，关系代词)
这些关系词和用法可以帮助你构建准确和流畅的定语从句。

记住，在选择关系词时，要根据先行词的性质和从句中的功能来确定适当的关系词。