# [time noun] khi in Vietnamese (B2)

> To say "the day when" or "the moment when", Vietnamese puts the time word straight before the sentence that describes it.

Source: https://vietnamesebuddy.com/grammar/khi-ma-when-relative/ · Updated 2026-10-17

**when / why (after a noun)**

Pattern: `[time noun] + (mà) + [sentence] · [time noun] + khi + [sentence] (written) · lý do / nguyên nhân + vì sao / tại sao + [sentence]`

Level: B2 (our estimate) · Register: formal

Typed without accents: khi, ly do vi sao, ly do tai sao

To say "the day when" or "the moment when", Vietnamese puts the time word straight before the sentence that describes it. In speech nothing joins them: *Hôm em tới, anh đi vắng*, the day you came, I was out. *Lúc*, *hồi* and *hôm* all work this way.

Writing likes a link. *Mà* after the time word adds weight and marks where the description starts: *thời điểm mà nhiều doanh nghiệp chuyển sang bán hàng trực tuyến*, the moment when many businesses moved to selling online. Papers also use *khi mà*: *trong bối cảnh khi mà chi phí tăng*, at a time when costs are rising. Treat *khi* after a noun as written style, and leave it out in conversation.

"The reason why" works the same way: *lý do* or *nguyên nhân* (cause), then *vì sao* or *tại sao*, then the sentence: *lý do vì sao sinh viên bỏ học*, the reasons why students drop out. Here *vì sao* isn't asking anything; it introduces what the reason explains. You'll meet it mainly in papers and reports. News writers often prefer a shorter link, *nguyên nhân khiến* (the cause that makes): *nguyên nhân khiến dự án chậm*.

In speech, people rarely say "the reason why" at all. They say *vì vậy* (so): *Vì vậy em nghỉ việc.* For "the place where", the same idea uses *nơi*.

## Examples

- Năm 2020 là thời điểm mà nhiều doanh nghiệp buộc phải chuyển sang bán hàng trực tuyến. — 2020 was the moment when many businesses were forced to move to selling online.
- Vào những ngày mà nhiệt độ lên tới 38 độ, nhu cầu dùng điện tăng vọt. — On days when the temperature reaches 38 degrees, demand for electricity soars.
- Nghiên cứu này tìm hiểu lý do vì sao sinh viên năm nhất dễ bỏ học. — This study looks at the reasons why first-year students are prone to dropping out.
- Báo cáo chưa làm rõ nguyên nhân vì sao dự án chậm tiến độ. — The report has not made clear why the project is behind schedule.

## Compare

- [Where (the place in which): nơi (mà)](https://vietnamesebuddy.com/grammar/noi-where-relative/): where (relative)
- [Which, that (the noun as object): noun + mà + sentence](https://vietnamesebuddy.com/grammar/relative-ma/): which / that / who(m)

Previous in Things, counting and numbers: [The beautiful, the good: cái + adjective](https://vietnamesebuddy.com/grammar/cai-nominal/)
Next in Things, counting and numbers: [The entire (formal): toàn, toàn bộ](https://vietnamesebuddy.com/grammar/toan-whole-formal/)

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