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從屬連詞用于過去分詞前的用法

當過去分詞或過去分詞短語用作狀語時，有時根據語義的需要可以在其前加上相應的連詞，如when, while, as, after, before, till, until, if, unless, although, though, even if, even though, once等。如：

If accepted for this post, you will be informed next week. (=If you are accepted for this post, you will be informed next week.) 如果你被接受擔任這個職位，將在下周通知你。

When heated, ice will be changed into water. (= When it is heated, ice will be changed into water. 冰加熱時，就會變成水。

Once seen, it can never be forgotten. (= Once it is seen, it can never be forgotten. 這東西一旦看到一次，就永遠也忘不了。

Unless (it is) changed, this law will make life difficult for farmers. 除非加以修改，否則這條法律將給農民的生活造成困難。

Although built before the war, the engine is still in perfect order. (=Although it was built before the war, the engine is still in perfect order.) 盡管是戰前制造的，這臺發動機仍然處于良好狀態。

Once exposed to light, the film will be damaged and get useless.(= Once exposed to light, the film will be damaged and get useless.) 一旦曝光，膠卷就要被損壞而不能用了。