一、副词 once 作“一次”、“一遍”、“一倍”讲
Ms. Liao has been to Tianjin once or twice. 廖女士去过一两次天津.
Not once has he done as I have asked. 他从来没有一次（即一次也没有）按我要求去做过。（句首使用否定的状语就要求后面的主谓语倒装。）
Once two is two. 一二得二。
They go on a trip once in a while（或 way）. 他们偶尔出去旅行。
Let’s hear you sing once more（或 once again). 让我们听你再唱一次。
Just for once（或 For this once, For that once, For once) she arrived on time. 她只有这一次准时到达了。
He had driven home drunk once too often —— this time he got stopped by the police. 他又一次酒后开车回家，但是这一次被警察拦住了。（once too often 本意是“过分频繁，超过一次、多于一次、又一次”。）
Once and again, Jenny said, "The Internet is playing an important role in the national economy." 詹妮再三地说：“互联网在国民经济中起着重要的作用。”
二、副词 once 作“一度”、“曾经”讲
I once thought that the answer to this question was correct. 我一度认为这个问题的答案是正确的。
He remembered to have once carried out this research work。他记得曾一度进行过这种科研工作.（once 说明作宾语用的不定式完成时。）
We visited the once capital of that country. 我们访问了那个国家的旧都。(这里 once 转为形容词，但仍含有“一度的”意思.)
三、连词 once 作“一旦”、“一经”讲，连接时间或条件从句，但有时连接一个省略句，省略与主语相同的主语或 it 及谓语 be 的相应形式
Once you begin, you must continue. 一经开始，你就必须继续下去。
Once（或 When once）it is gone, perhaps you will never get it back. 一旦失去什么，你也许永远找不回来。
Once put into operation, computers will play an important role. 计算机一运行，就将起重要作用。
四、“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
Such fish once taken out of the water will soon die. 如果一旦将这样的鱼从水中取出，它就马上死去。（=If once such fish is taken out of the water，it will soon die.)
The circuit once closed will soon put on the light. 电路一闭合就立即亮灯。(= If oncethe circuit is closed, it will…)
五、和 once 连用的其它常见短语
They were determined to end the matter once and forever [或 once (and) for all]. 他们决心一劳永逸地（或终于）结束这件事。
Once upon a time, there was an honest young man named Sidney. 从前有个诚实的青年，名叫锡德尼。
a. I must tell you the latest news at once. 我必须马上告诉你最新消息。
b. His talk was at once interesting and instructive. 他的讲话既有趣又有教育意义。a. All at once he lost his temper. 他突然发脾气。
b. The soldiers turned round and ran away all at once. 士兵们向后转并且一起跑步离开。
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