总共有五种情况：
1．当强调职业时。例如：
His daughter became a doctor after graduation, which he liked very much.他的女儿大学毕业之后当了医生。这是他非常喜欢的职业。
Her mother is a teacher, which is most glorious under the sun.她的妈妈是个老师，老师的职业是天底下最光荣的职业。
2．当先行词是婴儿或小孩时。例如：
The baby which is lying there has been crying for about twenty minutes.躺在那儿的那个婴儿已经哭了大约二十分钟了。
This is the very child which came here five minutes ago.这就是五分钟之前来这儿的那个小孩。
3．当强调品德或素质时。例如：
Professor Wang is a gentleman which our headmaster isn’t.王教授很有君子风度，我们的校长却没有。
4．当强调人的性格，身份和地位时。例如：
Last month I went to Beijing only to find my friend was not the man which he had been.上个月我去了北京，结果发现我的朋友不是像过去那个老样子。
Her father is different from the man which I thought him to be.他父亲不是我曾认为的那种人。
He talked like a teacher which he hardly was.他谈起话来像个老师，可他根本就不是。
5．当集体名词被看作一个整体时。例如：
We should take a great pride in the Chinese people which has a long history, a rich culture and glorious revolutionary tradition.我们应该为中国人民感到骄傲自豪。这是个有悠久历史、灿烂文化和光荣传统的民族。
希望对你有所帮助！
参考资料：高中语法书
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