before与when的区别（急）SeveralweekshadgonebybeforeIrealizedthepaintingwasmissing.中的before为什么不能换成when?

Question

before与when的区别（急）
Several weeks had gone by before I realized the painting was missing.中的before为什么不能换成when?

Answer 1

拆句子先
several weeks had gone by before I realised
the paiting was missing.
before 在.之前,此句意思为:画在我感觉到的几个星期前就丢了,表达之前,我的迟钝,
如果用 when 首先 when 当.时候 接时间点,several weeks 是个时间段,
其次 表达不出我事后的感觉