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by the time I came back they had climbed up..

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by the time I came back they had climbed up..
by the time I came back they had climbed up ten metres
现在完成时的结构不是has/have加动词过去分词么?为什么不用have climbed?
by the time I came back they had climbed up..
climbed up动作发生在came back 之前
came back 已经是过去时态(by the time到...时候)
过去完成时表示“过去的过去”
实际上,多数用法都有前后动作的时间对比,或暗含对比