问题描述：
not until的过去完成时
有这样两个句子：
1.The young man did not wake up until the bed had struck the ground.
2.I had not understood the problem until he explained it.
哪一个正确?具体讲讲原因呗.
有这样两个句子：
1.The young man did not wake up until the bed had struck the ground.
2.I had not understood the problem until he explained it.
哪一个正确?具体讲讲原因呗.
问题解答：
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