定语从句中的关系代词的作用是代替先行词,在从句中作先行词的成分,所以先行词在从句中作什么成分关系代词就作什么成分,如

The book that /which I bought yesterday is very interesting.(关系代词作宾语)

The book that /which was written by him is very interesting.(关系代词作主语)

The boy who is standing at the door is our monitor.(关系代词作主语)

The man whom you were talking with just now is Tom's father.(关系代词作宾语)