once a week
每周一次
I saw him once and never again.
我见过他一回，以后再也没见过。
I once lived in London.
我曾经在伦敦住过。
All at once the answer came to him.
他突然想到了答案。
I have to go home at once.
我必须马上回家。
He was busy writing a story, only stopping once in a while to smoke a cigarette.
他忙着写故事，只是偶尔停下来抽根烟。
Just for once let me make my own decision.
这一次就让我自己做决定吧。
Go on, lend me the bicycle, just this once.
好啦，把自行车借给我，就这一次。
The old couple have been married for 40 years and never once have they quarreled with each other.
那对老夫妇已经结婚40年了，彼此从未吵过架。
Tell me just once again.
你就再告诉我一次吧。
Let’s settle this matter once and for all.
我们把这个问题彻底解决了吧。
I have been there once or twice.
我去过那儿一两次。
Once you form a bad habit, it’s very difficult to get rid of it.
一旦养成了坏习惯，你就很难改掉它。