Not only did he hear it, but he saw it as well.
句中的 did 是什么意思？
当 never, hardly, little, seldom, nor, rarely, scarcely, no, not 等否定词以及 only + 状语位于句首时，句子的主语和谓语要实行部分倒装，即：
否定性副词(或 not + 名词) + 助动词 + 主语 + 实义动词
如果句子本身不带助动词,就要添加一个助动词 do(does, did):
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They not only rob you, they smash everything too. → Not only do they rob you, they smash everything too. 他们不仅抢劫你，而且还砸烂所有的东西。
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Little does he care about what others think. 他根本不在乎别人怎么想。
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Only then did he understand it. 只有那时他才明白。
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Not until Madame Curie discovered radium did people have an idea of radioactivity. 直到居里夫人发现了镭，人们才知道什么是辐射。
请与其他助动词对比：
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Scarcely have I met such a nice girl. 我几乎从未遇见过这么好的女孩。
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Only in this way can you master English. 只有用这种方法,你才能掌握英语。请做下面的英语试题:
The doctor talked about his research for an hour, but not a word John understood. (上海高 A B C D
考,96-F-8)