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.

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

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