once可以用作副词和连词。现就其做连词和副词以及常见的五个短语进行总结。
1.once做副词表示“仅一次；一次”。比如：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.她每半年才探望一次父母。I remember meeting him once somewhere.我记得在什么地方见过他一次。
once做副词表示“曾经”。比如：He once lived in Zambia.他曾在赞比亚生活。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。
once表示“每隔……一次”。比如：Lung cells die and are replaced about once a week. 肺细胞约每周死亡、更新一次。
2.once作为连词可以连接复合句。其意思是“一旦……就；一旦；当……时候”。比如：
We didn't know how we would cope once the money had gone.一旦钱花光了，我们就不知道该怎么办了。The water is fine once you're in!你一旦下了水，就会觉得水里挺舒适。
The decision had taken about 10 seconds once he'd read a market research study.他读完一份市场调研报告后，10秒钟就做出了决定。
3.由once构成的常见短语有all at once、at once、once more等五个短语，它们也在试卷中常常出现的。
* all at once指“突然，骤然，忽然” 。比如：All at once there was someone knocking on the door.突然有人敲门。All at once she lost her temper.她突然大发脾气。
*at once有两个常见意思：“立即，马上；同时”。比如：Come here at once!马上到这里来！I have to go at once.我得马上走。
I can't do two things at once.我不能同时做两件事。Don't all speak at once!大家不要同时讲！
*once(and)for all意思是“一劳永逸地，永久地”。比如：We have to resolve this matter once and for all.我们必须彻底解决这件事情。
The days when we had to import oil are gone once and for all.我们不得不进口石油的日子已一去不复返了。
*once more(once again)意思是“再一次”。比如：
Once again the train was late.火车又一次晚点了。Let me hear it just once more. 让我再听一遍。I hit him once more for luck.我无故又打了他一下。
*once in a while指“偶尔；有时”。比如：He comes to our room for a chat every once in a while.他偶尔来我们房中闲谈。
Your body, like any other machine, needs a full service once in a while.你的身体就像其他机器一样，偶尔也需要一次全面保养。
