we worked ten years ago 是factory 的定语（句）即.的工厂.是由where引导的典型的定语从句!先行词是地点（工厂）,或用in which/which.in.所以不能填第二个（that）.that也可引导定语从句但它的先行词须是物或人,或先行词又被大量的定语修饰时用.如 this is the book that you gavt me yeatday.中的that 引导物（书）且在从句中是宾语也可省略.如把 it was.that 去掉后 句意不完整即“我们做过、工作过,我第一次遇见他”没有地点原因什么的,所以不完整.,we worked ten years ago in the factory,I met him for the first time.这又是一种句式,比较散.是介词短语（in the factory ）做地点状语.that i met him for the first time is the factory where we worked ten years ago .这是系表结构复合句,它主语是从句（一句话）,表语是定语从句( where 指工厂,在从句中坐地点状语).现在英语强调简化.总之,只要语法对头,可以千变万化.本人也在学习水平有限,相互学习啊.