▲once用作副词有两个意思：

1. 一次，一回

I told him what to do just once. 我只给他讲了一次该做什么。

We write once a month. 我每月写一封信。

I’ll help you once but then you must do it by yourself. 我只帮你一次，但以后你就得自己做了。

Once he made bold to ask her to accompany him to a party. 有一次他鼓起勇气要求她陪他去参加一个晚会。

2. 一度，从前，曾经

Once there stood an old church on the hill. 山丘上曾经有一座古教堂。

This novel was once very popular but nobody reads it today. 这本小说曾经很受欢迎，但现在没有人读了。

▲once upon a time 是习语

通常用于儿童故事的开头，意思是“从前，很久以前”。如：

Once upon a time there was a princess. 从前有一个公主。

Once upon a time there lived a man known by the name of Joe Beef. 从前有个人名叫乔•比夫。

‘Once upon a time,’ he began, ‘there was a man who had everything.’“很久很久以前，”他开始说道，“有这么一个人，他应有尽有。”

Once upon a time in a faraway land there lived a princess in a big castle. 从前，在很远的地方，有一位美丽的公主住在一座很大的城堡里。