关系代词which的用法
在非限定性定语从句中which的用法，只宜用which, 不宜用that.
Beijing, which is the capital of China, is a very beautiful city.

在限定性定语从句中which和that在指代物的时候常常可以通用，但是有时只宜用which
而不用that
i. 关系代词前面有介词的时候
This is the hotel in which you will stay.
ii. 如果有两个定语从句，其中的一句的关系词是that, 另一句宜用which.
Let me show you the novel that I borrowed from the library which was newly
open to us.

3. 关系代词与介词，关系代词的省略
a) 关系代词和介词
介词放在关系代词的前面时，介词短语只能用which代物，和用whom代人
This is the hero of whom we are proud.
I want to find the very pen with which I wrote that letter.

当介词放在句子的末尾时，可用that/which代物，that/whom/who来作为介词的宾语，
且这个做介词宾语的关系代词往往忽略
This is the hero that (who/whom/忽略) we are proud of.