当先行词在从句中是作主语、宾语的时候,应该用关系代词.
反之,如果是作状语,那么就用关系副词
比如：
She works in the factory which lies in the north of the city.
这里的factory在后面的从句中是作主语的,所以用which引导从句.
She works in the factory where his father worked .这里的factory作从句的地点状语,所以用关系副词.
但是,关系副词还可以用介词+关系代词来代替,这个你知道吧?