who is the boy that is swimming in the pool?
在游泳池里游泳的那个男孩是谁?
Beijing,which is the capital of China,is a very beautiful city.
北京,中国的首都,是一座非常漂亮的城市
She is the girl who (whom) I met at the party.
她是那个我昨天在party上遇见的女孩
This is the book which i used before.
这就是我以前用的那本书