Once… then… (一旦…就/便)

Structure 一旦 + condition，就 / 便 + consequence

一旦 (yī dàn) sets up a hypothetical condition or a future moment that, once met, immediately triggers a consequence: "once…, as soon as…, should… happen". It is followed by 就 or, in more cultured register, by 便 (biàn), the literary counterpart of 就. 一旦决定了，就不要再犹豫 = "once you've decided, don't hesitate again"; 一旦发现问题，便立即上报 = "as soon as a problem is found, it is reported at once". It carries a nuance of warning or automatic causal chain, common in notices, regulations and argumentative prose.

Examples

一yī旦dàn决jué定dìng了le，就jiù不bù要yào再zài犹yóu豫yù。
Once you've decided, don't hesitate any more.
一yī旦dàn发fā现xiàn问wèn题tí，便biàn立lì即jí上shàng报bào。
As soon as a problem is detected, it is reported at once.
信xìn任rèn一yī旦dàn失shī去qù，就jiù很hěn难nán重chóng新xīn建jiàn立lì。
Once trust is lost, it is very hard to rebuild.
一yī旦dàn签qiān署shǔ合hé同tóng，双shuāng方fāng便biàn须xū履lǚ行xíng各gè自zì的de义yì务wù。
Once the contract is signed, both parties must fulfill their respective obligations.

Levels are indicative: they follow the standard Chinese Grammar Wiki (AllSet Learning) progression, not an official HSK 3.0 mapping. We never assign levels by eye.

This point and the other 370, with audio on every example, inside Wanyu.