一经…便/就…: once, as soon as
复句 / complex sentences
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
一经 ("as soon as it has been…") means that once an action happens, a result follows immediately and automatically. Written register: regulations, notices, press.
The verb after 一经 is often passive in meaning and disyllabic: 发现、发布、推出、确认、查实. The second part has 便 or 就. Typical: 一经发现，立即处理 ("any violation will be dealt with immediately").
Examples
这款手机一经推出，便受到了年轻人的欢迎。
Zhè kuǎn shǒujī yìjīng tuīchū, biàn shòudào le niánqīngrén de huānyíng.As soon as it was launched, this phone was a hit with young people.
虚假信息一经发现，平台就会立即删除。
Xūjiǎ xìnxī yìjīng fāxiàn, píngtái jiù huì lìjí shānchú.Once false information is found, the platform deletes it immediately.
合同一经签订，双方都必须遵守。
Hétong yìjīng qiāndìng, shuāngfāng dōu bìxū zūnshǒu.Once signed, the contract binds both parties.
Common mistakes
✗ 一经 for a routine: ✗ 我一经下班就回家 → ✓ 我一下班就回家。
✗ 一经 + common monosyllable: ✗ 一经看 → ✓ 一经发现 / 一经使用。