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ADV
一次；一回
If something happens once, it happens one time only.
例句
I met Wilma once, briefly...
我曾经和威尔玛有过短短的一面之缘。
Since that evening I haven't once slept through the night...
从那天晚上之后，我没有睡过一次安稳觉。
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ADV
（与后接day, week, month等的a连用）（每…）一次，（每…）一回
You use once with 'a' and words like 'day', 'week', and 'month' to indicate that something happens regularly, one time in each day, week, or month.
例句
Lung cells die and are replaced about once a week...
肺细胞约每周新老更替一次。
We arranged a special social event once a year to which we invited our major customers.
我们每年举办一次专门的社交活动，邀请重要客户出席。
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ADV
（与后接day, week, year等的every连用）（每隔…）一次，（每隔…）一回
You use once with 'every' and words like 'day', 'week', and 'year' to indicate that something happens a specified number of times and on a regular basis.
例句
The patient was seen for follow-up visits once every three months...
这位病人每三个月接受一次随访就诊。
My daughter comes to visit me once every fortnight.
我女儿每两周来看我一次。
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ADV
曾经，一度（状况如此）
If something was once true, it was true at some time in the past, but is no longer true.
例句
The culture minister once ran a theatre...
这位文化部长曾经经营过一家剧院。
I lived there once myself, before I got married...
我结婚前曾独自在那儿住过。
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ADV
曾经（做过某事）
If someone once did something, they did it at some time in the past.
例句
I once went camping at Lake Darling with a friend...
我曾和一位朋友在达令湖畔露营。
We once walked across London at two in the morning...
我们曾在凌晨两点步行穿过伦敦。
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CONJ-SUBORD
一旦…（就…); 一…（就…）
If something happens once another thing has happened, it happens immediately afterwards.
例句
The decision had taken about 10 seconds once he'd read a market research study...
在他看过一份市场调研报告后大概10秒钟就作出了决定。
Once customers come to rely on these systems they almost never take their business elsewhere...
顾客一旦依赖上这些系统，几乎就不会光顾其他商家。
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PHRASE
突然；忽然
If something happens all at once, it happens suddenly, often when you are not expecting it to happen.
例句
I feel terribly sleepy all at once...
我突然觉得特别困。
All at once there was someone knocking on the door.
忽然有人敲门。
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PHRASE
立即；马上
If you do something at once, you do it immediately.
例句
I have to go, I really must, at once...
我得走了，我真的必须马上就走。
Remove from the heat, add the parsley, toss and serve at once...
端离炉子，加上欧芹，颠起翻面后就立刻上桌。
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PHRASE
同时；一起
If a number of different things happen at once or all at once, they all happen at the same time.
例句
You can't be doing two things at once...
不能一心二用。
No bank could ever pay off its creditors if they all demanded their money at once...
如果所有的储户同时要求提取存款，没有哪家银行能办得到。
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PHRASE
就此一次；破例一次
For once is used to emphasize that something happens on this particular occasion, especially if it has never happened before, and may never happen again.
例句
For once, dad is not complaining...
爸爸这次破天荒地没有抱怨。
His smile, for once, was genuine.
只有这一次，他真诚地笑了。
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PHRASE
又一次；再次
If something happens once again or once more, it happens again.
例句
Amy picked up the hairbrush and smoothed her hair once more...
埃米拿起发梳，再次将头发梳理整齐。
Once again an official inquiry has spoken of weak management and ill-trained workers.
官方调查又一次提到了管理不力与工人缺乏训练的问题。
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PHRASE
彻底地；一劳永逸地
If something happens once and for all, it happens completely or finally.
例句
We have to resolve this matter once and for all...
我们必须彻底解决这一问题。
If we act fast, we can once and for all prevent wild animals in Britain from suffering terrible cruelty.
如果我们迅速采取行动，就可以一劳永逸地使英国的野生动物免受残忍的虐待。
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PHRASE
偶尔；间或
If something happens once in a while, it happens sometimes, but not very often.
例句
Earrings need to be taken out and cleaned once in a while.
耳环要偶尔取出来清洗一下。
Once in a while she phoned him.
她间或会给他打个电话。
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PHRASE
一两次；几次
If you have done something once or twice, you have done it a few times, but not very often.
例句
I popped my head round the door once or twice...
我有几次头撞到了门上。
Once or twice she had caught a flash of interest in William's eyes...
有那么一两次，她发现威廉的眼睛里流露出一丝感兴趣的神色。
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PHRASE
（常用于儿童故事的开头）从前，很久以前
Once upon a time is used to indicate that something happened or existed a long time ago or in an imaginary world. It is often used at the beginning of children's stories.
例句
'Once upon a time,' he began, 'there was a man who had everything.'...
“很久很久以前，”他开始说道，“有这么一个人，他应有尽有。”
Once upon a time, asking a woman if she has a job was quite a straightforward question.
从前，问一位女性是否有工作是个相当简单直接的问题。