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on one occasion only; one time 仅一次；一次 I've only been there once. 我只去过那里一次。 He cleans his car once a week. 他每周洗一次汽车。 She only sees her parents once every six months. 她每半年只探望一次父母。 For optimum effect, these drops are used once daily. 为了达到最佳效果，这些滴剂每天使用一次。 I had met Lou once before. 我以前见过娄一次。 Last year the region received rain only once. 去年这个地区只下过一次雨。 - (informal)
He only did it the once. 这种事儿他仅仅干过一次。 Rome is a city you should visit at least once in your life. 罗马是一座你一生中至少应该去一次的城市。 They meet once a week at a Paris café. 他们每周在巴黎咖啡馆见一次面。
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at some time in the past 曾；曾经 I once met your mother. 我曾经见过你母亲。 He once lived in Zambia. 他曾在赞比亚生活过。 This book was famous once, but nobody reads it today. 这本书曾名噪一时，但现在却无人问津。 She remembers once travelling with her father to Kilkenny. 她记得有一次和父亲一起去基尔肯尼旅行。 I famously once said that I was a fighter and not a quitter. 我有一句名言，我是一个斗士，不是一个半途而废的人。 He once told an interviewer that his biggest regret was missing his father's funeral. 他曾告诉一位采访者，他最大的遗憾是错过了父亲的葬礼。 It was once thought to be the best hotel in Europe. 它曾被认为是欧洲最好的酒店。
used in negative sentences and questions, and after if to mean ‘ever’ or ‘at all’ （用于否定句、疑问句和 if 后）曾，根本 used with a preposition or another adverb to mean ‘as soon as somebody/something is/was’ 一…就；一旦；当…时候 Once inside the gate, Sam hurried up the path. 一进大门，萨姆就急忙跑上小路。
Word OriginMiddle English ones, genitive of one. The spelling change in the 16th cent. was in order to retain the unvoiced sound of the final consonant.
Idioms
all at once
suddenly 突然；骤然；忽然 all together; at the same time 一起；同时
at once
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immediately; without delay 立即；马上 at the same time 同时
(just) for once | just this once
going once, going twice, sold (especially North American English)
(British English also going, going, gone)
said by an auctioneer to show that an item has been sold （拍卖师用语）一次，二次，成交
once again | once more
once a…, always a…
once bitten, twice shy
- (saying)
after an unpleasant experience you are careful to avoid something similar 一朝被蛇咬，十年怕井绳
once (and) for all
once in a blue moon
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very rarely 极少地；难得地；破天荒地
(every) once in a while
occasionally 偶尔地；间或 Everybody makes a mistake once in a while. 每个人都偶尔会犯错误。
once or twice
once too often
once upon a time