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by the time+过去时间，主句过去完成。by the time+将来时间，主句将来完成时。
by the time有两个意思
1)到...为止
如果从句中的谓语动词是一般过去时，那么主句中的谓语动词通常用过去完成时，表示“截止到从句动作发生时，主句动作已经完成，即过去的过去”
He had left by the time we reached home.
当我们到家的时候他已经走了。
2.如果从句中的谓语动词为一般现在时/或现在完成时（表示将来），那么主句中的谓语动词常为将来完成时，表示“截止到将来某一时间为止，主句动作将已经完成”
By the time you get back，I shall have finished the work。
到你回来时，我将已经把活做完了。
2)当……的时候
如果只是说过去某个时候的状况，用过去时，而不用完成时。
By the time we arrived the meeting was over.
我们到达时，会议已结束了。
by the time双语例句
By the time you get there the meeting will be over.
等你到了那里的时候，会议就该结束了。
By the time I arrived on the scene, it was all over.
我来到现场时，一切都已结束。
By the time this gets into print, they'll already have left the country.
这篇东西发表的时候，他们将已经离开这个国家了。
She was tired and out of sorts by the time she arrived home.
她回到家里，又累又烦。
By the time we arrived, someone had grabbed all the good seats.
我们到达时，所有的好位子都给人占了。