2 sentences for 一下子 (yī​xià​zi — quickly)

1. 他一下子就把作业做完了。

   tā yī​xià​zi jiù bǎ zuò​yè zuò​wán le.

   He finished his homework in no time.

   Source: Dong Chinese Dictionary

2. 他一下子钻进人群中不见了。

   tā yī​xià​zi zuān​jìn rén​qún zhōng bù​jiàn le.

   He suddenly dove into the crowd and disappeared.

   Source: Dong Chinese Dictionary