The factory in which/where his father worked has closed. The factory which was built in 1978 has closed. 第一个例子,the factory 是先行词,从句中的work为不及物动词,the factory做从句的状语,所以可以用关系副词where 表地点,也可以用in which (因为work in the factory) 第二个例子,the factory 是先行词,从句缺主语,the factory 做从句的主语,用关系代词(which/that) 所以分清先行词在从句中所做的句子成份很重要(做主语、宾语用关系代词;做状语用关系副词) 关系代词:that/which/who/whom/whose 关系副词:when,why,where 介词+which 灵活运用就OK了.