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同学好：
定语从句只用who的情况

1.只指人，并且在从句中作主语的情况

The man who is a new one has rented a flat in downtown of New York for a long time.
新来的这个人在纽约市中心租了一栋公寓，有一段时间了。（the man指人并在句中作主语）

2.在非限定性定语从句中作主语，并指人时

He feel honored to work with Mr.Li, who has good reputation in the business.
他感到很荣幸和李先生一起工作，李先生在业界有着良好的声誉。

3.先行词是god, angel,fairy等词时，用who不用that

In the oil painting, the state of angels who smiled at each other delivered the artist`s emotional.
在油画中，天使互相微笑的状态传递了艺术家的情感。


定语从句中只能用which，不能用that的几种情况

1.在介词后面的关系代词用which而不能用that。即“介词+which（代物）”

The picture for which he paid $1,000 was once owned by a duke.
他花了1000美元买下的画曾为一名公爵所拥有。

2.在非限定性定语从句中不能用that。

Crusoe's dog, which was very old now, became ill and died.
克鲁索的狗，现在已经很老了，病死了。

3.有两个定语从句，其中一个关系代词已有which，另一个可以用that。相反，如果其中一个为that，另一个可以用which。

Edison built up a factory which produced things that had never been seen before.
爱迪生办了一个工厂，生产过去从未见过的东西。


祝学习愉快～