關系副詞(when、where、why)引導的定語從句,它們後面的句子主幹完整，只缺狀語，它們都可以等於介詞+ which。也就是說關系副詞和關系代詞可以相互轉化，二者的選擇主要看該關系詞在從句要充當的句法成分。
1、When?
指時間,在定語從句中作時間狀語。其先行詞是表時間的名詞(如:time, day, week, month, etc.)。但並非先行詞為"時間名詞",就用when引導。當關系詞充當定語從句的狀語時,用when;當關系詞充當主語或者賓語時用which或that?引導。
I still remember the day when my brother joined the army.(作狀語，when可以轉化為on which)
I still remember the days which/that we spent together. (作spent的賓語)
2、Where?
指地點在定語從句中作地點狀語。其先行詞是表示地點的名詞(如:place, school, room, etc.)。同理，先行詞是"地點名詞"時,定語從句可用where引導,還可用which/that引導，取決於該引導詞在從句中的成分。
This is the factory where he worked last year. (作狀語, where可以轉化為in which)
This is the park which/that they visited last year. (作visited的賓語)
3、Why?
指原因在定語從句中作原因狀語,?先行詞通常為reason。同理，先行詞為表原因的名詞時,還要根據引導詞在從句中成分決定用why還是which/that。
The reason why he didn't come to school was that he was ill(作狀語).
I don't believe the reason (that/which) he gave me. (作give的賓語)
He asked me the reason that can explain my success. (作explain的主語)
擴展資料
關系副詞when,where和why可以用介詞+which的形式替換。
1、when=介詞+which
I will never forget the day_____ we met for the first time.?
解析：先行詞是the day，後面的從句恢復到完整句為：we met for the first time on the day.因此，on the day作狀語用關系副詞when代替，如果用介詞+which，就改成on which。答案為when或on which。
2、where =介詞+which
This is the farm______ we worked when we were young.
解析：先行詞是the farm，後面的從句恢復到完整句為：we worked on the farm when we were young. 因此，on the farm可以用關系副詞where代替，如果用介詞，就改成on which。
3、why= for which
I don't know the reason ______he did it.
解析：先行詞是the reason，後面的從句恢復到完整句為：he did it for the reason. 因此，for the reason可以用關系副詞why代替，如果用介詞，就改成for which。