视频概述

1. adv.一次

He cleans his car once a week.

他一周洗一次车

adv.仅一次

I've only been there once.

我只去过那里一次

2. adv.曾经

The 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

3. conj.

一旦……就……

We didn't know what could we do once the money had gone.

一旦钱花光了，我们就不知道该怎么办了。