9.once or twice一两次；几次
She had been to London once or twice before.她以前去过伦敦一两次。
10.once upon a time<文>(in children's stories)从前；很久以前
Once upon a time there were three princes.很久很久以前, 有三位王子。
<口>(used nostalgically)(用于谈论已不存在的状况, 尤指过去的状况比现在的好)想当年
Once upon a time, I wanted to change the world.想当年, 我曾想改变这个世界。