定语从句中在哪些情况下用关系副词?

定语从句中在哪些情况下用关系副词?*I will never forget the day when I first met you on the seashore.*I will never forget the days which/that we spent together in Xueyao Middle School.为什么上句用when而下句用which?**This is the hospital where my mother works.**This is the hospital which/that we visited the day before yesterday.***The reason why he did not go to school is that he was ill.***This is the reason which/that he gave his boss in the office .还有这四句,为什么不用相同的词引导?关系副词都可换成“介词+关系代词”的形式吗？