We are studying sentences. They contain adjective clauses.
同样道理，我们用which代替they，来指代sentences，告诉对方我们正在学习什么类型的句子。
We are studying sentences which contain adjective clauses.
我们正在学习带有形容词性从句的句子。
which可以指代由单个名词或代词构成的先行词：
a. Anger is a wind which blows out the lamp of the mind.
愤怒是吹灭心灵之灯的风。
b. Perhaps it is human to appreciate little that which we have and to long for that which we have not.
或许，“不惜已获，贪求不得” 是人类的共同弱点吧。（先行词是that，which指代单个代词。）
which可以指代一个短语：
c. He likes climbing mountains, which is a good exercise. （先行词是主句的一部分climbing mountains）
which还可以指代前面整个主句：
d. Tom was late, which surprised me.
汤姆迟到了，这让我很吃惊。
e. Jane tore up my photo, which upset me.
简撕毁了我的照片，这令我很伤心。
f.Jane wore her swimming things in the office, which shocked her boss a great deal.
简把泳装穿到办公室来，这着实让她的老板吃了一惊。
g. Tom ran all the way from the station, which I thought was incredible（难以置信）.
我真不敢相信汤姆竟然一路跑到了车站。