先行词是表示时间的词如time,year, day等且在定语从句中做时间状语,用关系副词when
I will never forget the day when I joined the Party.
先行词是表示地点的词如place,factory,hospital等且在定语从句中做地点状语,用关系副词where
This is the factory where my father works.
先行词是reason,且在定语从句中做原因状语,用关系副词why
That's the reason why he looks so old.