# Một khi: once in Vietnamese (B2)

> Một khi opens a condition, like English "once": once the first thing happens, the second follows, and usually can't be undone.

Source: https://vietnamesebuddy.com/grammar/mot-khi-once/ · Updated 2026-10-17

**once**

Pattern: `Một khi + [sentence], …`

Level: B2 (our estimate) · Register: formal

Typed without accents: mot 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.

## Examples

- Một khi dữ liệu cá nhân bị rò rỉ, người dùng gần như không thể kiểm soát được nữa. — Once personal data has leaked, users have almost no control over it.
- Một khi đã mất niềm tin, khách hàng rất khó quay lại. — Once trust is lost, customers rarely come back.
- Một khi các tuyến metro hoàn thành, lượng xe máy vào trung tâm sẽ giảm đáng kể. — Once the metro lines are finished, the number of motorbikes coming into the centre will fall considerably.
- Một khi được áp dụng rộng rãi, mô hình này có thể giúp nông dân giảm chi phí. — Once widely adopted, this model could help farmers cut their costs.

## Compare

- [Whenever (every time): hễ (cứ) … là](https://vietnamesebuddy.com/grammar/he-whenever/): whenever

Previous in Time and when things happen: [No sooner … than: vừa … đã, chưa … đã](https://vietnamesebuddy.com/grammar/vua-da-no-sooner/)
Next in Time and when things happen: [Originally, by nature: vốn (là), vốn dĩ](https://vietnamesebuddy.com/grammar/von-originally/)

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