"which" 是一个关系代词，用于引导 定语从句、 宾语从句或 主语从句。具体用法如下：

定语从句：

which引导的定语从句通常用来修饰、限定或说明一个名词或名词短语。例如：

I like the book which you lent me.

The house, which I bought last year, is very beautiful.

宾语从句：

which引导宾语从句时，可以在从句中作主语、宾语或表语。例如：

She asked me which book I had read.

He told me the news which surprised me.

主语从句：

which引导主语从句时，通常指代前文提到的某个事实或情况。例如：

Which is the best choice？

There is a problem which we need to solve.

根据上下文的不同，"which" 引导的从句类型和作用也会有所变化。希望这些信息对你有所帮助。