关系代词在英语语法中用于引导定语从句，连接主句和从句，并在从句中代替先行词，起到连接作用。关系代词主要有以下几种：
指人的关系代词
who
whom
whose
指物的关系代词
which
that
特殊的关系代词
as
than
but
whosever
whoever
whomever
whosever
of which
of whom
where
when
why
how
wherein
whereby
等
关系代词在定语从句中充当主语、宾语或定语。例如，在句子 "The book that I borrowed is interesting." 中，"that" 是关系代词，它在定语从句 "I borrowed" 中充当主语，指代先行词 "the book"