Vietnamese cognition and speech predicates do not conjugate, but they select sharply different complements. Biết can take a fact, a person someone recognizes or knows of, or an action that someone knows how to perform; nghĩ can introduce an opinion; nói presents speech, while bảo and nói với explicitly connect it to an addressee. Choosing the frame carries more grammatical information than changing the shape of the predicate ever would.
English encourages learners to search for tense endings and to translate prepositions such as to and about one by one. Vietnamese instead combines an invariant predicate with a noun, a whole clause, a verb phrase, or an addressee phrase. This page covers thought and communication. Literal perception belongs on Perception Predicates, and physical displacement belongs on Motion Predicates.
Choose the complement by meaning
| Intended meaning | Productive frame | Example core |
|---|---|---|
| know a fact; know who/where | biết + noun / clause | biết chuyện đó; biết cô ấy ở đâu |
| know how to perform an action | biết + verb phrase | biết lái xe |
| understand content | hiểu + noun / clause | hiểu vấn đề; hiểu vì sao... |
| think that, hold an opinion | nghĩ (là/rằng) + clause | nghĩ rằng cô ấy đúng |
| think about, consider | nghĩ về + noun | nghĩ về đề nghị đó |
| say content | nói + content | nói là sẽ đến |
| speak to someone | nói với + person | nói với tôi |
| tell/instruct someone | bảo + person + clause/VP(informal/neutral) | bảo tôi đợi |
| ask someone a question | hỏi + person + question/content | hỏi tôi mấy giờ |
These frames are tendencies with conventional extensions, not interchangeable slots. In particular, biết làm describes learned ability, whereas có thể làm describes possibility or capacity in the circumstances. See Biết + Verb for Learned Ability for that contrast.
With a person, biết chị Lan normally means knowing who Lan is, recognizing her, or knowing of her. Personal acquaintance is more clearly expressed with quen. English know covers both relations, so this lexical distinction is easy to miss.
Tôi biết chị Lan là ai, nhưng tôi không quen chị ấy.
I know who Lan is, but I don't know her personally.
Knowing, understanding, remembering, and forgetting
Biết presents information as part of someone's knowledge. A following embedded question keeps its Vietnamese question word in place; there is no English-style inversion.
Tôi biết chị ấy làm ở đâu.
I know where she works.
Anh có biết cuộc họp bắt đầu lúc mấy giờ không?
Do you know what time the meeting starts?
Em gái tôi biết lái xe nhưng chưa có bằng.
My younger sister knows how to drive but does not have a license yet.
The unchanged biết works with tôi, anh, or em gái tôi. Time and status come from the complement and words such as chưa, not from person or tense inflection.
Hiểu focuses on grasping meaning, logic, or a reason. Knowing a fact does not guarantee understanding it.
Tôi biết quy định đó, nhưng tôi chưa hiểu vì sao nó cần thiết.
I know that rule, but I still don't understand why it is necessary.
Nhớ can mean “remember,” “keep in mind,” or “miss,” depending on its complement and context. Quên can take the thing forgotten or a verb phrase describing an omitted action.
Nhớ gửi tài liệu cho tôi trước chiều mai nhé.
Remember to send me the documents before tomorrow afternoon.
Tôi quên mang chìa khóa nên phải quay lại văn phòng.
I forgot to bring my keys, so I had to go back to the office.
In these instructions and reports, gửi and mang directly follow the cognition predicate. English inserts to, but Vietnamese does not copy it with đến or another destination word.
Thinking, considering, and deciding what you believe
For an opinion, nghĩ commonly takes a clause directly or after là/rằng. Rằng is especially associated with careful writing and planned speech (formal/written); là is extremely common as a colloquial complement linker (informal/neutral). Neither is always obligatory.
Tôi nghĩ mai trời sẽ mưa.
I think it will rain tomorrow.
Tôi nghĩ là mình nên nói với chị ấy trước.
I think we should speak to her first.
Báo cáo kết luận rằng biện pháp này chưa đạt hiệu quả mong muốn.
The report concludes that this measure has not achieved the desired effect.
The third sentence uses rằng in formal written reporting. In everyday speech, tôi nghĩ là... or a direct clause after tôi nghĩ... is more natural. With a topic rather than a proposition, nghĩ về means “think about.”
Tôi cần thêm thời gian để nghĩ về đề nghị của anh.
I need more time to think about your proposal.
Do not add về before a whole opinion merely because English says think about whether.... Vietnamese may use nghĩ xem có nên... không “consider whether one should...” when active deliberation is intended.
Saying, speaking, telling, and asking
Nói can present manner or content without naming an addressee: nói nhỏ “speak quietly,” nói tiếng Việt “speak Vietnamese,” nói là... “say that....” An addressee is normally introduced by với.
Chị Lan nói với tôi là cuộc họp đã đổi sang thứ Sáu.
Lan told me that the meeting had been moved to Friday.
Anh nói chậm lại được không? Tôi nghe chưa rõ.
Could you speak more slowly? I didn't hear clearly.
Bảo + person + clause or verb phrase is a compact and highly frequent way to report a message, request, or instruction (informal/neutral). Its force comes from context: it may translate tell, ask, or say to.
Mẹ bảo tôi về sớm vì tối nay nhà có khách.
My mother told me to come home early because we have guests tonight.
Anh Minh bảo là anh ấy đang đến.
Minh says that he is on his way.
In the first, tôi is both the addressee of bảo and the understood subject of về. In the second, no addressee is expressed and là introduces the reported message. More formal reporting often uses cho biết rằng “state/inform that” (formal) or phát biểu rằng “state that” (formal).
Hỏi directly takes a person and then the question content; it does not need an equivalent of English to.
Cô ấy hỏi tôi ở đây có Wi-Fi không.
She asked me whether there was Wi-Fi here.
Notice the embedded yes/no frame có... không. The original question could be Ở đây có Wi-Fi không? Embedding it after hỏi tôi does not trigger subject–auxiliary inversion.
The same form, different discourse times
Hôm qua anh ấy nói là không biết; sáng nay anh ấy lại nói là đã hiểu rồi.
Yesterday he said he didn't know; this morning he said again that he understood it now.
Both occurrences of nói have the same form. Hôm qua and sáng nay locate them; đã hiểu rồi presents understanding as newly established. English marks said and understood morphologically, but Vietnamese uses time expressions and aspectual words.
The broader system uses equally distinct lexical frames:
| Clause | What licenses the predicate |
|---|---|
| Tôi ăn cơm. | ăn takes an object |
| Cô ấy là giáo viên. | là classifies with a noun |
| Trời lạnh. | lạnh is already a property predicate |
| Tôi có một câu hỏi. | possessive có |
| Ở đây có Wi-Fi. | existential có |
| Tôi nghĩ là cô ấy đúng. | nghĩ takes a proposition |
Do not insert là before every adjective because English uses be: Tôi nghĩ là trời lạnh contains là as a colloquial clause linker after nghĩ, while the embedded property clause remains trời lạnh, not trời là lạnh.
A change of complement changes the message
A: Chị đã nói với Nam chưa? B: Rồi. Tôi nói là cuộc họp bắt đầu lúc tám giờ, nhưng anh ấy nghe thành chín giờ. A: Thế chị bảo anh ấy kiểm tra lại tin nhắn nhé.
A: Have you spoken to Nam yet? B: Yes. I said the meeting starts at eight, but he heard it as nine. A: Then tell him to check the message again.
Nói với Nam names the addressee; nói là... supplies content. Bảo anh ấy kiểm tra packages an addressee and requested action in one frame. If the first question were merely Chị đã nói Nam chưa?, it would sound incomplete or nonstandard for the intended “speak to Nam,” because ordinary nói does not take the addressee exactly as English tell does.
Common Mistakes
1. Copying English “tell someone” with nói + person.
❌ Tôi nói anh ấy là mai nghỉ.
Unnatural for the intended ‘I told him tomorrow is a day off’: the addressee frame is missing.
✅ Tôi nói với anh ấy là mai nghỉ.
I told him that tomorrow is a day off.
Use nói với + person or choose bảo + person in an ordinary conversational instruction.
2. Adding đến as a translation of English infinitival “to.”
❌ Tôi quên đến gọi cho chị ấy.
Incorrect for ‘I forgot to call her’: đến would express motion/arrival, not an infinitive marker.
✅ Tôi quên gọi cho chị ấy.
I forgot to call her.
Quên takes the action verb phrase directly.
3. Inserting là inside a property clause.
❌ Tôi nghĩ là trời là lạnh.
Incorrect: the second là cannot serve as a general copula before lạnh.
✅ Tôi nghĩ là trời lạnh.
I think it is cold.
The first là links the opinion to its clause (informal/neutral). Lạnh then predicates the weather directly.
4. Using nói about a language with an English preposition.
❌ Cô ấy nói bằng tiếng Việt rất giỏi.
Awkward if the intended meaning is simply that she speaks Vietnamese well.
✅ Cô ấy nói tiếng Việt rất giỏi.
She speaks Vietnamese very well.
Nói + language is the ordinary lexical frame. Bằng is possible when explicitly contrasting the medium used, not as a mandatory copy of English in.
5. Treating đã as a past-tense ending required on every report.
❌ Hôm qua anh ấy nói-đã là bận.
Incorrect: đã is an independent aspect word, not a suffix attached to nói.
✅ Hôm qua anh ấy nói là anh ấy bận.
Yesterday he said that he was busy.
The time phrase is enough to locate nói in the past. Add independent đã only where its realized/prior viewpoint contributes to the meaning.
Key Takeaways
An uninflected Vietnamese predicate can still demand a precise argument structure. Use biết/hiểu + noun or clause, biết/nhớ/quên + verb phrase, nghĩ (là/rằng) + proposition, nghĩ về + topic, nói với + addressee, and bảo/hỏi + person + content. Time expressions, aspect words, complement type, and discourse context do the work that English often spreads across tense morphology, infinitives, complementizers, and prepositions.
Now practice Vietnamese
Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.
Start learning Vietnamese →Related Topics
- Perception PredicatesB1 — Use Vietnamese predicates for seeing, hearing, smelling, tasting, and feeling with the complement frames and aspectual readings they naturally select.
- Motion PredicatesB1 — Express movement, destination, direction, manner, and deictic perspective with Vietnamese motion predicates and predicate sequences.
- Giữ: Keep, Hold, and MaintainA2 — Use giữ for retaining control, holding something in place, preserving a state, and honoring commitments or confidentiality.
- Được for Permission and OpportunityA2 — Use preverbal được for being allowed or getting the chance to act, and distinguish it from ability and explicit authorization.