Several weeks had gone by I realized the painting was missing.

问题描述:

Several weeks had gone by I realized the painting was missing.

几个礼拜过去了,我才意识到我把画件弄丢了。

当我意识到那幅画丢失了的时候,几个星期已经过去了。

很明显,前面半句是过去完成时,表示过去的过去,后面半句是过去时.因此前面的动作发生在后面的之前
所以要用before,而不能用since,就是这个道理

过了几个星期,我才发现油画丢失了