once表达曾经可以放在句首
也可以用Once upon a time
Once they lobbied hard to join nato .
他们曾经极力游说以加入北约。
Globular clusters once ruled the milky way .
球状星团曾经统治着银河系。
Do this about once a week .
油大概每周上一次。
I was in this exact situation once .我曾经历过一模一样的情形。
We cannot take back words once spoken .
话一旦说出我们不可能收回。
Once upon a time there was a beautiful girl who lived in a big castle.