yī dàn
毁于一旦。
Huǐ yú yī dàn.
Destroyed in a single day/moment.
相处三年，一旦离别，怎么能不想念呢？b）用于未然，表示“要是有一天”：理论一旦为群众所掌握，就会产生巨大的物质力量。
Xiāngchǔ sān nián, yī dàn líbié, zěnme néng bù xiǎngniàn ne? b) Yòng yú wèirán, biǎoshì "yàoshi yǒu yī tiān": Lǐlùn yī dàn wèi qúnzhòng suǒ zhǎngwò, jiù huì chǎnshēng jùdà de wùzhì lìliàng.
After three years together, once separated, how can one not miss each other? b) Used for hypothetical situations, meaning "if one day": Once theory is grasped by the masses, it will generate tremendous material force.