核心问题
Only when a great deal more information has been obtained, will it be possible to plan a manned trip to Mars. (New Concept English, Book 3, L.9)
只有在获得了大量资料后，才有可能计划载人的火星之行。
我不明白主句中的主语(it)与will为什么要倒装？
词典解答
在 only + 副词、介词短语或状语从句位于句首时，句子主语与谓语要进行部分倒装（即主语与助动词倒装）。这是一条固定的语法规则。
Only when a child grows up does he understand his parents' intention. (IME) 只有当儿童长大成人时才会理解父母的意图。
(参见 only then)
练习题
请做下面的英语试题：
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Only in this way ____ to make improvement in the operating system. A. you can hope B. you did hope C. can you hope D. did you hope(上海高考,03春-39)
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____ by keeping down costs will Power Data hold its advantage over other companies. A. Only B. Just C. Still D. Yet(全国高考浙江卷,06-9)