Khi introduces a clause that locates another event in time: Khi tôi đến, mọi người đang ăn tối “When I arrived, everyone was having dinner.” Depending on context, it can mean when, as, or sometimes whenever. Vietnamese does not change the form of khi for past, present, or future; time reference comes from context, time expressions, aspect markers, and the relationship between the two events.
This page covers neutral temporal clauses with khi. The more specific orderings trước khi “before” and sau khi “after” have a dedicated page, while trong khi and its simultaneous or contrastive readings are treated separately on Trong Khi and Simultaneity or Contrast.
The basic pattern
A khi clause can precede or follow the main clause:
Khi tàu đến ga, chúng tôi sẽ gọi cho anh.
When the train arrives at the station, we will call you.
Tôi không nghe máy khi anh gọi vì đang tắm.
I did not answer when you called because I was showering.
The second example puts the main point first and adds its time with final khi; vì separately gives the reason. Fronting the time frame yields Khi anh gọi, tôi đang tắm nên không nghe máy “When you called, I was showering, so I did not answer.” Clause order is therefore not only grammar; it helps package the discourse.
| Intended meaning | Useful frame | Typical cue |
|---|---|---|
| one past occasion | khi + event, main event | hôm qua, lúc đó, đã |
| future time | khi + event, main clause + sẽ | ngày mai, đến lúc đó, sẽ |
| recurring time | khi + situation, main habit | thường, hay, mỗi lần |
| background plus highlighted event | khi + background, thì + event | đang...thì... |
| explicit “whenever” | mỗi khi / hễ...thì... | repeated correlation |
Initial khi clauses set the scene and are often followed by a pause in speech or a comma in writing. Final khi clauses add the time after the main point. Neither order is inherently more formal.
Tôi gặp chị Mai khi đang đợi xe buýt.
I met Mai while I was waiting for the bus.
The omitted subject of đang đợi is normally controlled by tôi, the salient subject of the main clause. If Mai was waiting, say so: Tôi gặp chị Mai khi chị ấy đang đợi xe buýt.
One occasion, a recurring situation, or a future event
Context determines whether khi refers to one occasion or a general pattern:
Khi tôi mở cửa sáng nay, một con mèo đang ngồi trước hiên.
When I opened the door this morning, a cat was sitting on the porch.
Khi trời trở lạnh, bà tôi thường nấu cháo gà.
When the weather turns cold, my grandmother usually makes chicken porridge.
Khi em đến Hà Nội vào tuần sau, anh sẽ ra sân bay đón em.
When you arrive in Hà Nội next week, I will meet you at the airport.
English requires special care with tense in future when-clauses: it normally says “when you arrive,” not “when you will arrive.” Vietnamese has no equivalent inflectional rule. Sẽ can occur where a future prediction needs emphasis, but a future time phrase often makes it unnecessary inside the khi clause. In the last example, vào tuần sau anchors đến, while sẽ marks the planned result.
For an unambiguously repeated correlation, mỗi khi means “whenever/every time.”
Mỗi khi căng thẳng, tôi đi bộ một vòng quanh hồ.
Whenever I feel stressed, I walk around the lake.
Plain khi could work, but mỗi khi directs the listener to a repeated pattern. Hễ...thì... is stronger: it presents one situation as a reliable trigger for another and belongs to the later page on habitual conditions.
Aspect and event shape
Khi itself does not say whether an event is completed or ongoing. Aspectual material supplies that perspective:
Khi tôi đến, cuộc họp đã bắt đầu.
When I arrived, the meeting had already begun.
Khi tôi đến, mọi người đang thảo luận ngân sách.
When I arrived, everyone was discussing the budget.
Tôi đang đọc sách thì nghe tiếng cửa mở.
I was reading when I heard the door open.
The first đã places the meeting’s start before the arrival. The second đang presents the discussion as ongoing at the arrival. The third uses đang...thì..., a compact and highly natural event frame: đang supplies a background process, and thì introduces the event that occurs against it. Although English translates thì here with when, thì is not interchangeable with khi in every temporal clause.
Shared and different subjects
When both clauses share a subject, Vietnamese commonly omits it in the khi clause if the interpretation is obvious:
Khi về đến nhà, tôi mới nhận ra mình để quên chìa khóa ở văn phòng.
When I got home, I realized that I had left my keys at the office.
The understood subject of về đến nhà is tôi. With different participants, state both subjects:
Khi khách đến, nhân viên lễ tân sẽ kiểm tra tên trong danh sách.
When guests arrive, the receptionist will check their names on the list.
Dropping khách would make the receptionist the most likely understood subject. Vietnamese permits zero arguments, but omission is not a license for reference to drift.
Khi inside the larger connector system
Different linkers answer different questions:
| Model | Question answered |
|---|---|
| Tôi biết rằng anh ấy đến. | What do I know?—a complement |
| người mà tôi gặp | Which person?—a relative modifier |
| nếu trời mưa thì ở nhà | Under what condition?—a conditional |
| vì bận nên tôi về sớm | Why and with what result?—cause and result |
| tuy bận nhưng tôi vẫn đến | What expected result is overridden?—a concession |
| tôi đến sớm để chuẩn bị | For what intended outcome?—a purpose clause |
| khi trời mưa, tôi thường ở nhà | At what time/situation?—a temporal frame |
The border between time and condition can be subtle. Khi trời mưa, tôi thường ở nhà presents rain as a recurring time frame. Nếu trời mưa thì ở nhà treats rain as a condition for the instruction “stay home.” Future khi normally presupposes or schedules the event, whereas nếu leaves it conditional:
Khi anh đến, gọi cho tôi nhé.
When you arrive, call me.
Nếu anh đến thì gọi cho tôi nhé.
If you come, call me.
The first expects the arrival; the second does not commit to it.
Temporal clauses can occur inside complements and relatives:
Tôi nhớ rằng khi còn nhỏ, anh ấy rất sợ chó.
I remember that when he was little, he was very afraid of dogs.
Người mà tôi gặp khi đi công tác đã gửi tài liệu cho tôi.
The person I met on a business trip sent me the documents.
In the first example, formal narrative or careful argument uses rằng to make the complement boundary explicit; reported speech can nest khi in the same way after nói rằng or báo rằng. Everyday speech more naturally says Tôi nhớ hồi nhỏ anh ấy rất sợ chó, with hồi nhỏ and no complementizer. In the second, khi đi công tác belongs inside the mà relative. Do not add rằng merely because a sentence is long.
Dialogue: time or condition?
A: Khi chị gặp giám đốc, chị sẽ đưa tài liệu này cho ông ấy chứ? B: Tôi chưa chắc sẽ gặp ông ấy. Nếu gặp thì tôi sẽ đưa.
A: When you meet the director, will you give him this document? B: I’m not sure I’ll meet him. If I do, I’ll give it to him.
A’s khi treats the meeting as expected. B corrects that assumption with nếu, and the bare gặp is recoverable from context. The connector changes the commitment, not merely the translation.
A: Khi nào mình bắt đầu? B: Khi mọi người đến đủ thì bắt đầu.
A: When do we start? B: We’ll start when everyone has arrived.
Khi nào asks for a time. In B’s answer, final thì cleanly marks the transition from the time frame to the main event; it is helpful but not obligatory. This use of thì reflects Vietnamese topic-comment organization.
Register and clause chains
Khi is neutral in speech and writing. Lúc can introduce a time clause too and is especially common in speech; vào lúc is more noun-like and often introduces a clock time. Một khi often means “once” in the conditional sense and should not be substituted blindly for ordinary khi.
In narrative, Vietnamese may omit repeated temporal linkers once sequence is obvious:
Khi chuông reo, cô giáo bước vào, đặt sách lên bàn rồi nhìn cả lớp.
When the bell rang, the teacher came in, put her books on the desk, and looked at the class.
The single khi frames the chain; rồi orders later actions. Repeating khi before every verb would imply separate frames and make the narrative labored. Bare juxtaposition works because one actor and a natural sequence keep the logic transparent.
Common Mistakes
1. Using nếu when the event is expected
❌ Nếu tàu đến ga, chúng tôi sẽ xuống.
Wrong for the intended scheduled meaning: this makes the train’s arrival sound conditional.
✅ Khi tàu đến ga, chúng tôi sẽ xuống.
When the train reaches the station, we will get off.
Use khi when the event is the expected time anchor; use nếu when its occurrence is genuinely conditional.
2. Copying English future marking into both clauses
❌ Khi ngày mai anh sẽ đến, tôi sẽ đưa chìa khóa.
Awkward: ngày mai and the construction already establish the future, while sẽ is misplaced inside the fronted time clause.
✅ Ngày mai, khi anh đến, tôi sẽ đưa chìa khóa cho anh.
Tomorrow, when you arrive, I will give you the key.
Vietnamese can use sẽ inside a khi clause in suitable predictive contexts, but it is not mechanically required by English will.
3. Dropping a different subject
❌ Khi đến, nhân viên lễ tân sẽ kiểm tra hộ chiếu của khách.
Ambiguous for ‘when the guest arrives’: the omitted subject is likely interpreted as the receptionist.
✅ Khi khách đến, nhân viên lễ tân sẽ kiểm tra hộ chiếu của họ.
When the guests arrive, the receptionist will check their passports.
Name both participants when subject continuity changes.
4. Overusing rằng in conversation
Tớ nhớ rằng khi cậu đến thì mọi người đang ăn.
I remember that when you arrived, everyone was eating. (Grammatical, but overly explicit for this casual recollection.)
✅ Tớ nhớ lúc cậu đến, mọi người đang ăn.
I remember that when you arrived, everyone was eating.
The casual repair uses lúc and juxtaposition. Formal rằng remains appropriate when a report needs a visible complement boundary.
5. Mismatching temporal and causal connectors
❌ Khi đường tắc nên tôi đến muộn.
Incorrect as one paired frame: khi opens a time clause, while nên announces a result from a cause.
✅ Vì đường tắc nên tôi đến muộn.
Because traffic was congested, I arrived late.
If time is intended, say Khi tôi đến thì đường đang tắc “When I arrived, traffic was congested.” Choose the logical relation before choosing the linker.
Key Takeaways
- Khi supplies a time frame; it does not itself mark past, ongoing, completed, or future aspect.
- The khi clause may precede or follow the main clause, while initial position is especially useful for scene-setting.
- Context distinguishes one occasion, a recurring situation, and a future event; mỗi khi makes repetition explicit.
- Khi treats an event as the temporal anchor, whereas nếu presents it as a condition.
- Explicit linkers guide complex prose; natural clause chains can use omission and juxtaposition when time and reference remain clear.
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Start learning Vietnamese →Related Topics
- Trước Khi and Sau KhiB2 — Order two events explicitly with trước khi and sau khi while controlling subject reference, completion, and clause scope.
- Trong Khi and Simultaneity or ContrastB2 — Use trong khi for overlapping events and for whereas-style contrasts, with context and clause structure guiding the reading.
- Vì...Nên Cause and ResultB2 — Link an everyday reason to its consequence with vì...nên, or omit one half when context already makes the relation clear.
- Nếu...Thì ConditionalsC1 — Build open Vietnamese conditionals with nếu...thì and interpret time, probability, and counterfactuality from context rather than verb inflection.
- How Vietnamese Locates Events in TimeA1 — Understand how context, time expressions, event type, and markers such as đã, đang, sẽ, rồi, từng, mới, vừa, chưa, sắp, and vẫn work together.