which在定语从句中是关系代词，在从句可以作主语或者宾语。当which作宾语时可以省略。
例句：
1、A plane is a machine which can fly.
which在从句中作主语，指代先行词machine.
2、The book which I bought last Sunday is very intetesting.
which在从句中作宾语，指代先行词book.关系代词作宾语可以省略。
3、Tom was late for school again， which made his teacher very angry.
which在从句中作主语，指代整个主句.
