- on one occasion or for one time only一次：
they deliver once a week
他们一星期送一次货
as noun the once informal he'd only met her the once.〈非正式〉 他只见过她一次。
- usu. with negative or if at all; on even one occasion (used for emphasis)丝毫, 一点; 一次也（用于强调）：
he never once complained
他一次也没抱怨过
if she once got an idea in her head you'd never move it.
她只要一有想法, 你就休想让她改变主意。
- at some time in the past; formerly曾经; 从前：
Gran had once been a famous singer.
奶奶曾是个著名歌唱家。
- multiplied by one乘以一：
once two is two.
二乘以一得二。
- as soon as; when一旦; 一…就：
once the grapes were pressed, the juice was put into barrels.
葡萄一被挤压, 葡萄汁就被装进桶里。
all at once
- without warning; suddenly突然：
all at once the noise stopped.
突然, 吵闹声停止了。
- all at the same time同时：
a lot of beans are ready all at once.
同时准备好了很多豆子。
at once
- immediately立刻; 马上：
I fell asleep at once.
我马上就睡着了。
- simultaneously同时：
computers that can do many things at once.
可以同时做很多事的电脑。
for once（或this once）
- on this occasion only, as an exception就（这）一次; 破例一回：
I hope you'll forgive me this once.
我希望你会原谅我这一次。
once a -, always a -
- proverb a person cannot change their fundamental nature〈谚〉做过一次…的人, 就会一直是…的人：
once a whinger, always a whinger.
抱怨过一次, 就会一直抱怨。
once again（或more）
- one more time又一次, 再一次。
once and for all（或once for all）
- now and for the last time; finally一劳永逸; 彻底地。
once and future
- denoting someone or something that is eternal, enduring, or constant永久的; 持久的; 持续的：
side two contains four once and future hit singles.
第二面有四首持续走红的单曲。
ORIGIN: 1950s: from T. H. White's Once and Future King (1958).
once bitten, twice shy
- 见BITE.
once（或every once）in a while
- from time to time; occasionally偶然; 间或。
once or twice
- a few times一两次。
once upon a time
- at some time in the past (used as a conventional opening of a story)从前（用于故事的开头）。
- formerly曾经：
once upon a time she would have been jealous, but no longer.
她曾经嫉妒心很强, 不过现在不了。