where可以是疑问副词、关系副词、融合型关系副词。

作为疑问副词，where可以引导特殊疑问句，例如，Where does he live? 语法上把这种特殊疑问句称为直接疑问句。where引导的特殊疑问句也可以作名词性成分，即在主句中作主语、宾语、表语等。此时where仍然是疑问副词，只不过问句不再用倒装语序，而用自然语序。传统语法把此时的where称为连接副词，以区别作为独立句子的问句和作为名词性从句的问句。但是现代语法不使用连接副词这个术语，仍然把where称为疑问副词，用作名词性从句的问句称为间接疑问句。I don't know where he lives.

作为关系副词，where引导定语从句（限制性或非限制性），修饰一个地点名词。例如，This is the factory where he works.

作为融合型关系副词，where可以引导名词性关系分句和副词性关系分句，前者在主句中作名词性成分，后者在主句中作地点状语。

引导名词性关系分句的where=the place where，即where把表示地点意义的先行词和关系副词where融为一体，其中隐含的先行词在主句中作名词性成分，where引导定语从句修饰先行词，where在从句中作地点状语或表语。例如，This is where he works.= This is the place where he works. where在从句中作状语。This is where he was yesterday. = This is the place where he was yesterday. where在从句中作表语。

引导副词性关系分句的where=in/at/to the place where, 即where把一个表示地点的介词短语和关系副词where融为一体，其中隐含的介词短语在主句中作状语，关系副词where引导限制性定语从句修饰介词的宾语，where在定语从句中作状语或表语。例如：Stay where you are.= Stay in the place where you are. where在从句中作表语。Put the book where you took it. = Put the book in the place where you took it. where在从句中作地点状语。