一旦 … ，就 … (once …, then …)

Cross the line, trigger the rest

• in case (sth happens)一旦yí dàn … ，then / just就jiù … = "once …, (then) …" — once the condition is met, the result follows.
• in case (sth happens)一旦yí dànto decide (to do something)决定jué dìng，就别jiù biéto regret后悔hòu huǐ = "once you've decided, don't regret it."
• in case (sth happens)一旦yí dàn marks a turning point after which things change.

in case (sth happens)一旦yí dàn + Condition，then / just就jiù …

• in case (sth happens)一旦yí dàn sets the trigger; then / just就jiù delivers what follows.

once一旦yí dàndecide决定jué dìng，then就jiùdon't别biéregret后悔hòu huǐ。

• in case (sth happens)一旦yí dànto decide (to do something)决定jué dìng，就别jiù biéto regret后悔hòu huǐ = "once you decide, don't regret it."

Recap

Read the full reference: HSK 5 grammar — Conditional statements.