once可以用作副词和连词。
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。
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。
