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问题: 一道选择

Several weeks had gone by_________I realized the painting was missing.
A.before B.since
为什么不选since,如何更改可以让since也符合条件?

解答:

如果用since引导的从句是过去时,主句应该是现在完成时.
Several weeks has gone by since I realized the painting was missing.
而用before,主句表示的是过去的过去,用过去完成时.你可以体会一下其中的意思.