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.
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.
【特别提示】你提问的第2个句子，时态正确。主句用过去完成时，完全符合“动作发生在过去，一直持续到过去某个时间点”的原则。句子真正的含义是：“这个问题我不明白，一直持续到老师讲解为止”。
2对
第2个是错的
第2个句子出现在新概念二册的14课,如果你买的是2011年7月第97次印刷版本之前的版本,书上写的恰好就是第2个句子.
但是后来的版本是更正了的,因为not...until 是 直到...才...的意思,比如 I didn't have supper until 10p.m.翻译过来是我直到10点才吃晚饭 而不是 我直到10点都还没吃晚饭.
所以当用NOT UNTIL 引导过去完成时的时候,UNTIL 后面接的动作（动1）要先于 NOT 后面接的动作（动2）发生,因为只有直到动1发生了,动2才发生,所以动1是先于动2的,所以动1我们用过去完成时.
第一句是对的,翻译过来就是,直到床碰到了地面他才醒过来,醒过来这个动作是发生在床碰到地面这个动作之后的,而醒过来这个动作用的一般过去时,所以碰地面这个动作就发生在过去的过去用过去完成时.
第二个是对的，过去完成时就是出现在发生在句中已有时间点（且对这个时间点的描述是过去式）之前发生的事。
所以第二句如果要用过去完成时应该是：the young man had not woken up until he struck the ground.
但是一般而言，until是不用特意强调这个时间的，因为until本身代表了事件发生顺序。
过去完成时例句：
I had been at the bus stop for 20 minutes when a bus finally came
He said he had worked in that factory since 1949.
Mr. Smith died yesterday. He had been a good friend of mine
I didn’t know a thing about the verbs, for I had not studied my lesson.
I returned the book that I had borrowed.
She found the key that she had lost.
He said that he had known her well
I thought I had sent the letter a week before
After he arrived in England, Marx worked hard to improve his English
When I woke up, it had already stopped raining.
We had hoped to be able to come and see you
It was the third time that he had been out of work that year
I had finished reading the novel by nine o'clock last night
We had learned over two thousand English words by the end of last term
They had planted six hundred trees before last Wednesday
I met Wang Tao in the street yesterday. We hadn't seen each other since he went to Beijing