1.I’ll never forget the days we worked together.

定语从句中,关系副词可以转换成介词加which；
when、why、where是关系副词,在从句当中做状语（时间、原因、地点）
而which、that、who等是关系代词,在从句当中做主、宾、表.
这句中,先行词是the days,在从句中做时间状语,故which不行
带进去就是we worked together in the days.