(1) The reason why/ for which he refused the invitation is not clear,

　　(2) From the year when/in which he was going to school he began to know what he wanted when he grew up.

　　注：1)介词后面的关系代词不能省略。

　　2)that前不能有介词。

　　3) 某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when ,where和why 互换。

　　This is the house in which I lived two years ago.

　　This is the house where I lived two years ago.

　　Do you remember the day on which you joined our club?

　　Do you remember the day when you joined our club?

　　This is the reason why he came late.

　　This is the reason for which he came late.