如何辨别that和which在定语从句中是做主语还是谓语?

先行词在从句中的成分决定引导词在从句中的成分。
I love the girl. The girl is a football player.the girl就是先行词，在从句里做主语。变成定语从句时who 就是主语。I love the girl who is a football player.
I bought a book.The book was very interesting.The book是先行词，在从句中做主语。变成定语从句时which就是主语。I bought a book which was very interesting.
I bought a book .He wrote the book last year. the book是先行词，在从句中做宾语。变成定语从句时which就是宾语。I bought a book which he wrote last year.
that在定语从句里，可以用来替代who,whom,which.

看他代替的是主语还是谓语

看看你的定语从句缺什么成分，那它们就做什么成分如
Football is a game which is liked by most boys.
这句中定语从句差的是主语，所以which是做主语
This is the pen which he bought yesterday.
这句中bought后面差一个宾语，所以这里的which做宾语

问你个问题，你懂主语和宾语是什么东东吗？如果不是很清楚的话，还是看看语法书吧

他们只能做主语或宾语 呵呵 给你个例子 我打开门 我是主语 打是谓语 门是宾语 你看他们在句子翻译过后的位置就行了 理解

你看它指代的是哪部分
被指代部分是什么成分，它就是什么成分的从句

后面没有介词的就是主语
还有注意是不是后面有动词，有就是主语