WHICH引导一个非限制性的定语从句，表示主句整句话的内容
例句：
It rained hard yesterday ，which perevented me from going to the park。
which用作关系代词，可引导限制性定语从句或非限制性定语从句，引导非限制性定语从句时which可以用整个主句作为其前行词，相当于and that，有时还可引导不定式短语从句。
例句： 1、It's hard to say which is better. 很难说哪一个好些。
2、Which are the best headache tablets? 哪些药片治头痛最好
3、Want to know which shops Bi Ren went to? 想知道碧仁在拍节目时去了哪些商店
which引导代替整个主句的句子有哪些
一 which可以引导定语从句， 修饰表示事物的先行词， 在定语从句中充当主语、 宾语或者表语。例如： He was reading a book， which was about war. 他正在读一本关于战争的书。（关系代词which在修饰book的定语从句中充当主语。）
He was reading a book， which he had bought from London. 他正在读一本书， 这本书是从伦敦买回来的。（关系代词which在修饰book的定语从句中充当宾语。
