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
：1、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）
2、that引导定语从句时，在从句中既可以作主语，也可以作宾语。如：
This is the dictionary that has helped me a lot in my English study. （作主语）
This is the dictionary that you are looking for. （作宾语）
3、who引导定语从句时，在从句中一般作主语。如：
He is the man who called me four times yesterday.
4、whom是who的宾格形式，在定语从句中只能作宾语。如：
She is the woman whom I served just now.
句中的whom因为是作宾语的，所以可以省略。
5、因为whose是所有格形式，表示所属关系，所以在实际使用中，whose后面会紧跟一个名词，才能引导定语从句。如：
She lives in a room whose?window opens to the north.