一旦

jī huì yī dàn cuò guò yǒng yuǎn bù huì zài lái

机会一旦错过，永远不会再来。

Once an opportunity slips away, it is gone for good.

yī dàn yǒu shì shuí huì bāng zhù nǐ ne

一旦有事，谁会帮助你呢？

If you have a problem, who will help you?

tā men xiāng ài duō nián yī dàn fēn bié nán miǎn sī niàn

他们相爱多年，一旦分别，难免思念。

After being together for years, they can't bear to part from each other.

huǐ yú yī dàn

毁于一旦

be destroyed in one day