The conjunction 一旦
The conjunction 一旦 
indicates that an action will happen if a certain condition is met.
We can also use 一旦 in the structure 一旦…就… .
indicates that an action will happen if a certain condition is met.
We can also use 一旦 in the structure 一旦…就… .Scheme
一旦
condition
(就)
Consequence
Examples
一旦发生事故，你要负责！
In case of an accident, you will be responsible for everything
yídàn fāshēng shìgù, nǐ yào fùzé
你要想好了，一旦选择了就不能放弃
You have to think carefully. Once you make a decision, it cannot be changed later
nǐ yào xiǎnghǎo le, yídàn xuǎnzé le jiù bù néng fàngqì
一旦我到了迈阿密就能找到工作
I can get a job as soon as I get to Miami
yídàn wǒ dào le mài'āmì jiù néng zhǎodào gōngzuò