英语中常用的关系代词有：which、that、who、whom、whose。根据这些代词在句中的句法功能，可以把它们分为主格、宾格和所有格三类，即：
主格：which、that、who
宾格：which、that、who、whom
所有格：whose
which引导定语从句时，在从句中既可以作主语，也可以做宾语。如：
Yesterday I lost the book which my English teacher had given me as a birthday gift. （作宾语）
Yesterday I lost the book which had been given to me by my English teacher as a birthday gift. （作主语）
