/wʌns ||; wʌns/adv conj 1 one time only; on one occasion 一次；一回: I've only been to France once. 我只去过一次法国。once a week/month/year 每周╱月╱年一次I visit them about once every six months. 我大约每隔六个月拜访他们一次。
2 at some time in the past; formerly 曾经；一度: This house was once the village school. 这幢房子从前是村里的学校。
3 as soon as; when 一…就；当…: Once you've practised a bit you'll find that it's quite easy. 稍稍练习之后，你会发觉那是很容易的。
all at once all at the same time or suddenly 同时；一起；突然；忽然:
People began talking all at once. 大家都同时开始说起话来。All at once she got up and left the room. 她突然站起来离开了房间。 at once 1 immediately; now 立刻；马上；现在: Come here at once! 立刻来这里！
2 at the same time 同时: I can't understand if you all speak at once. 如果你们都同时说起话来，我就听不明白。
just this once;(just) for once on this occasion only 只此一次:
Just this once, I'll help you with your homework. 我来帮你做家庭作业，不过只此一次，下不为例。 once again;once more one more time; another time 又一次；再一次:
Once again the train was late. 火车又一次晚点。Let's listen to that track once more. 我们再来听一遍那首歌。 once and for all now and for the last time 最后一次:
You've got to make a decision once and for all. 你得作出最后决定。 once in a blue moon(
informal 非正式) very rarely; almost never 罕有；差不多从不
once in a while sometimes but not often 偶尔；间或
once upon a time (used at the beginning of a children's story) a long time ago; in the past （用于儿童故事的开头）很久以前；从前:
Once upon a time there was a beautiful princess... 从前有一位美丽的公主…