问
Once on the boat, they were taken below and their chains were fixed to two bars that ran the length of the ship.
上句中的 once 是个什么词？起什么作用？
答
once 引导的从句,如同 if, when, while, until, although 引导的从句一样,有时可以省略主语和谓语或谓语的一部分,尤其是 be。因此,在英语里常常看到这样的结构:once (when, while, until) + 现在分词/过去分词/形容词/介词短语/副词:
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Take care when crossing the road. (省略了 you are) 过马路时要小心。
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Once opened, the contents should be consumed within three days. 一旦打开,其中的内含物应在三日内用完。
不难断定, Once on the boat 乃是 Once they were on the boat 的省略形式。省略形式比非省略形式要正式一些, 多用于书面语体(参见 ellipsis , when )。
练习
The research is so designed that once ____ nothing can be done to change it.