Once (when and if): một khi

Một khi opens a condition, like English "once": once the first thing happens, the second follows, and usually can't be undone. Một khi đã mất niềm tin, khách hàng rất khó quay lại, once trust is lost, customers rarely come back.

It starts the first half of the sentence. The second half follows after a comma, sometimes with thì. Đã often comes right after một khi to stress that the first step is complete: một khi đã quyết, once you've decided.

The difference from plain khi (when) is the consequence. Khi just places something in time. Một khi says the first thing changes the situation, and from then on the result is certain or hard to stop. It often carries a warning or a firm prediction.

You'll meet it in papers, opinion pieces and reports, and in careful speech when someone is making a point: Một khi nó đã quyết thì không ai cản được, once he's made up his mind, nobody can stop him.

In everyday conversation, đã … thì does a similar job: Đã làm thì làm cho tới, if you're going to do it, do it properly. Hễ … là (whenever) is for things that repeat; một khi is usually about one decisive change.