không

1. Basic Placement
Place không immediately before the verb or adjective you are negating.
• Tôi không biết. (I don't know.)
• Trà này không nóng. (This tea is not hot.)
If the sentence uses a tense or aspect marker like đã, sẽ, or đang, không must be placed after the marker (e.g., Tôi sẽ không đi. [I will not go.], Tôi đang không làm việc. [I am not working.]). For modal verbs like nên, không is typically placed before the modal verb (e.g., Tôi không nên đi. [I should not go.]). Note that for 'cannot', the standard and most common form is không thể rather than 'không có thể' (though the latter is occasionally heard), and placing không after có thể changes the meaning to 'might not' (e.g., Tôi có thể không đi. [I might not go.]).

In spoken Vietnamese, people frequently insert có before the adjective or verb to emphasize the negation in the present tense (e.g., Tôi không có buồn! [I am not sad!]). Notably, in Southern Vietnamese, không có + [verb] is also the standard, everyday way to negate past tense actions (e.g., Hôm qua tôi không có đi làm. [Yesterday I didn't go to work.]).

2. Asking and Answering Questions
To ask a Yes-No Question about present, future, or habitual actions, use the formula: [Subject] + (có) + [Verb/Adjective] + không? Note that if the main verb is có (to have), you should only use one có to avoid doubling it (e.g., Bạn có tiền không? [Do you have money?]). To ask a simple binary question about a past fact, you can use the same structure: [Subject] + có + [Verb] + không? (e.g., Hôm qua bạn có đi làm không? [Did you go to work yesterday?]). This contrasts with đã ... chưa?, which specifically asks if an action has been completed yet (e.g., Bạn đã ăn chưa? [Have you eaten yet?]).
To answer 'Yes', reply with Có or repeat the main verb/adjective (though for a có phải ... không? question, the correct 'Yes' is Phải or Đúng). For 'No', your answer depends on the question type: reply with Không (or Không + verb) for có ... không? questions, and Chưa (or Chưa + verb) for đã ... chưa? questions. Không is the default word for 'No' and can be used as a standalone answer (e.g., Không, cảm ơn. [No, thank you.]). Replying to a question with just Không or repeating the main verb with không (e.g., Không hiểu. [I don't understand.]) is grammatically correct but casual and can be blunt. To be polite to elders or strangers, you must add status markers, such as saying Dạ không or Không ạ. Note that to answer 'No' to a có phải ... không? question, you should use Không phải (Not correct / It is not).

3. Pronunciation Note
The word không is spoken with a flat, mid-level pitch (flat tone / thanh ngang). The kh- sound is a voiceless velar fricative, similar to the 'ch' in the Scottish 'loch' or German 'Bach', not a hard English 'k' or a standard English 'h'. Crucially, because it ends in -ông, you must close your lips and puff your cheeks slightly at the very end of the word; otherwise, it will sound like the English 'ong'. In casual speech, especially in the South, people often drop the 'k' entirely and just say hông.

4. Important Restrictions and Meanings
• When placed directly before a noun, không means 'without' or 'zero' (e.g., không đường [no sugar]). It can also mean 'empty' or 'nothing' in certain common contexts (e.g., tay không [empty hands], phòng không [empty room]). To say someone or something 'is not' a noun, you must use không phải là (is not) instead.
• To say you 'do not have' something, use không có + Noun (e.g., Tôi không có tiền. [I don't have money.]).
• Không can be used to negate past events when meaning 'did not'. However, you must use chưa (not yet) when answering đã ... chưa? questions about actions that might still happen (like 'Have you eaten?'). Answering with Không in these cases implies a hard refusal ('I am not eating' or 'I refuse to eat'), which is a common pitfall for learners.
• You cannot use không to tell someone not to do something (a command). For the imperative 'don't', you must use đừng (e.g., Đừng đi. [Don't go.]).

5. The Number Zero
Besides negation, không is also the Vietnamese word for the number zero (0), which you will frequently encounter in phone numbers and prices.

6. Common Phrases and Variations
• To express 'cannot' or 'not allowed to', use không thể or không được rather than just không.
• A very common phrase is không có gì, which means 'nothing' or is used as a polite 'you're welcome' in response to cảm ơn.
• In casual speech, especially in the North, you will often hear chẳng or chả used as direct substitutes for không (e.g., Tôi chẳng biết or Tôi chả biết [I don't know]).