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。)
which引导非限定性定语从句时有一个特殊用法, 即它所引导的非限定性定语从句修饰主句中更多的部分甚至整个主句(这时被定语从句所修饰的“先行词”要作广义理解), which仍在定语从句中作主语、 宾语或表语。例如:
He set free the birds happily, which was a celebration for his success. 他开心地把鸟放了, 这是对他成功的一种庆祝。
把非限定性定语从句“which was a celebration for his success”的先行词视为主句中的“the birds”显然不符合整句的语境, 被“which was a celebration for his success”所修饰的是整个主句, which所引导的这类特殊的非限定性定语从句对主句所叙述的情况进行某种意义的补充说明, which常可译为“这一点, 这件事”。