爱华网Once的用法
一、once作“一次”、“一遍”、“一倍”解：(adv)
1. We go to the plant once a day. 我们一天去工厂一次。
2. I did this once before, and I will do it again.我从前做过异辞，而我现在要再作一次。
3. He read this text only once yesterday.这个课文他昨天只读了一遍。
4. Once two is two. 一二得二。
二、once作“一度”、“曾经”解：(adv)
1. Once he learned some English. 他曾经学过一点英语。
2. I once thought that the answer to this questionwas correct. 我一度认为这个问题的答案是正确的。
3. There once lived an old scientist who wrote manyarticles. 从前有一个老科学家，他写了许多文章。
4. He remembered to have once carried out thisreaction. （once说明作宾语用的不定式完成时。）他记得一度进行过这种反应。
三、once作“一旦”、“一经”解，连接时间或条件从句。例如：(conj)
1. An object, once it is in motion, will persist inthat motion because of its inertia. 物体一经运动，由于惯性就将继续保持其运动。
2. Once you begin, you must continue.一经开始，你就必须继续下去。
3. Suppose there would be no friction. Oncestarted, a body would be kept in motion.假设不存在摩擦。物体一经起动，就将继续保持运动。（Once引出具有条件意味的时间状语从句，省略了与主句相同的主语及其相应的be的变化。两个would表示虚拟语气。）
4. Once (it is) gone, perhaps you will never get itback. 一旦失去什么，也许你再也找不回来。（句中Once还可用When once代之，意思相同。）
四、once连接一个省略句（省略与主句相同的主语或it及其相应的be）：(adv)
1. If all the impeding forces of gravitation andresistance could be removed, there is no reason why the ball, once(it is) in motion, should ever stop.如果一切起阻止作用的引力和阻力能加以除去，就没有理由认为，球一旦处于运动中会再停下来。
2. Once combined, nitrogen is no longer chemicallyinert. (=Once nitrogen is combined, it is not chemically inert anylonger.) 氮气一经化合，在化学上就不再是惰性气体了。
3. The magnesium has to be warmed to start a rapidreaction, but once started, large quantities of heat and lightenergy are liberated. (=…but once it is started, … )镁必须予以加热才能发生迅速反应，但反应已经开始，就有大量热和光能放出。
五、“once+过去分词”作后置定语时，可理解为“If once…”，具有条件从句含义。例如：
1. This rule once learned will never be forgotten.(=If once this rule is learned it will…)一旦学了这条规则，就永远不会忘记。
2. Such a thing once seen will always beremembered. (=If once such a thing is seen it will…)如果见到这种事情，就将永远记住。
3. A fish once taken out of the water soon dies.鱼从水中取出，就马上死去。
六、常用词组once（and）forall作“最后一次”、“只此一次”解以及带once的其他词组用法举例：
1. We solved once for all the difficult problem.我们终于解决了这个难题。
2. We overthrew the rule of the landlords once andfor all. 我们最后推翻了地主统治。
3. The oil worker comes to give us his lessons oncein a while. 这位石油工人偶尔给我们上课。
4. Once upon a time there was a river here.从前这里有一条河。
5. I want to search into the matter once more.我想再次调查那件事。
6. You must settle on(或upon) a rendezvous at once.你必须马上选定一个集合地点。
7. His talk was at once interesting andinstructive. 他的讲话既有趣又有教育意义。
-----http://blog.hjenglish.com/ariosto/articles/1446295.html
