从属连词用于过去分词前的用法
当过去分词或过去分词短语用作状语时，有时根据语义的需要可以在其前加上相应的连词，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.) 如果你被接受担任这个职位，将在下周通知你。
When heated, ice will be changed into water. (= When it is heated, ice will be changed into water. 冰加热时，就会变成水。
Once seen, it can never be forgotten. (= Once it is seen, it can never be forgotten. 这东西一旦看到一次，就永远也忘不了。
Unless (it is) changed, this law will make life difficult for farmers. 除非加以修改，否则这条法律将给农民的生活造成困难。
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.) 尽管是战前制造的，这台发动机仍然处于良好状态。
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，胶卷就要被损坏而不能用了。