3 sentences for 一下 (yī​xià — suddenly)

1. 木板太脆了，承受不住重压，一下就劈了。

   mù​bǎn tài cuì le, chéng​shòu bù zhù zhòng​yā, yī​xià jiù pǐ le.

   The board was too brittle to withstand the heavy pressure and split instantly.

   Source: Dong Chinese Dictionary

2. 他的脑袋转得很快，一下就想出了办法。

   tā de nǎo​dai zhuǎn de hěn kuài, yī​xià jiù xiǎng​chū le bàn​fǎ.

   His mind works quickly; he came up with a solution in an instant.

   Source: Dong Chinese Dictionary

3. 刚才还晴空万里，谁知道一下就下起了瓢泼大雨。

   gāng​cái hái qíng​kōng​wàn​lǐ, shéi zhī​dào yī​xià jiù xià​qǐ le piáo​pō​dà​yǔ.

   It was a clear, cloudless sky just a moment ago, then all of a sudden, it started pouring rain.

   Source: Dong Chinese Dictionary