when(=at,on,in,during which) (时间名词,时间状语)
e.g.I shall never forget the day when (on which) we moved into our new house.
where(=in,at,which) (地点名词,地点状语)
e.g.This is the place where (at which) Jack parks his car.
why(=for which) (指代原因,原因状语)
e.g.He didn't know the reason why (for which) he was dismissed.