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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...
我曾經(jīng)和威爾瑪有過短短的一面之緣。
Since that evening I haven't once slept through the night...
從那天晚上之后，我沒有睡過一次安穩(wěn)覺。
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ADV
（與后接day, week, month等的a連用）（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...
肺細(xì)胞約每周新老更替一次。
We arranged a special social event once a year to which we invited our major customers.
我們每年舉辦一次專門的社交活動，邀請重要客戶出席。
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ADV
（與后接day, week, year等的every連用）（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...
這位病人每三個月接受一次隨訪就診。
My daughter comes to visit me once every fortnight.
我女兒每兩周來看我一次。
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ADV
曾經(jīng)，一度（狀況如此）
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...
這位文化部長曾經(jīng)經(jīng)營過一家劇院。
I lived there once myself, before I got married...
我結(jié)婚前曾獨自在那兒住過。
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ADV
曾經(jīng)（做過某事）
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...
我曾和一位朋友在達(dá)令湖畔露營。
We once walked across London at two in the morning...
我們曾在凌晨兩點步行穿過倫敦。
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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...
在他看過一份市場調(diào)研報告后大概10秒鐘就作出了決定。
Once customers come to rely on these systems they almost never take their business elsewhere...
顧客一旦依賴上這些系統(tǒng)，幾乎就不會光顧其他商家。
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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...
我突然覺得特別困。
All at once there was someone knocking on the door.
忽然有人敲門。
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PHRASE
立即；馬上
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，我真的必須馬上就走。
Remove from the heat, add the parsley, toss and serve at once...
端離爐子，加上歐芹，顛起翻面后就立刻上桌。
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PHRASE
同時；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...
如果所有的儲戶同時要求提取存款，沒有哪家銀行能辦得到。
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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...
爸爸這次破天荒地沒有抱怨。
His smile, for once, was genuine.
只有這一次，他真誠地笑了。
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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...
埃米拿起發(fā)梳，再次將頭發(fā)梳理整齊。
Once again an official inquiry has spoken of weak management and ill-trained workers.
官方調(diào)查又一次提到了管理不力與工人缺乏訓(xùn)練的問題。
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PHRASE
徹底地；一勞永逸地
If something happens once and for all, it happens completely or finally.
例句
We have to resolve this matter once and for all...
我們必須徹底解決這一問題。
If we act fast, we can once and for all prevent wild animals in Britain from suffering terrible cruelty.
如果我們迅速采取行動，就可以一勞永逸地使英國的野生動物免受殘忍的虐待。
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PHRASE
偶爾；間或
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.
耳環(huán)要偶爾取出來清洗一下。
Once in a while she phoned him.
她間或會給他打個電話。
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PHRASE
一兩次；幾次
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...
我有幾次頭撞到了門上。
Once or twice she had caught a flash of interest in William's eyes...
有那么一兩次，她發(fā)現(xiàn)威廉的眼睛里流露出一絲感興趣的神色。
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PHRASE
（常用于兒童故事的開頭）從前，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，”他開始說道，“有這么一個人，他應(yīng)有盡有。”
Once upon a time, asking a woman if she has a job was quite a straightforward question.
從前，問一位女性是否有工作是個相當(dāng)簡單直接的問題。