Thr girl

Thr girl ________ is reading under the tree is my sister.为什么不填whom 而填who
2者有什么区别吗?

定语从句中,指人的先行词我们用who或者whom做连接词,who用来做从句的主语,whom用来做从句的宾语.
This is a nurse who wears a white skirt.
It's a policeman who gave us an way to the park.
He is a worker who works very hard.
上文中,连接词who都做下文的主语,此时我们用who是最为恰当的.而在连接词做下文的宾语时,更多的是用whom.
This is the person whom I just saw.
*This is the person I just saw whom.
She is a teacher whom I like best.
*She is a teacher I like whom best.
Tom is the driver whom we talked about.
*Tom is the driver we talked about whom.
上文三句中,whom都做从句的宾语.