⇒ 这座城堡竟然在一次大火中毁于一旦。 (Zhè zuò chéngbǎo jìngrán zài yī cì dà huǒ zhōng huǐ yú yīdàn.) The great fire destroyed this castle in a single day.
副
⇒ 一旦发生核战争，整个地球将会陷入灾难。 (Yīdàn fāshēng hézhànzhēng, zhěnggè dìqiú jiāng huì xiànrù zāinàn.) If atomic war were to break out, it would be disastrous for the whole world.