khi nào

In Vietnamese, where you place a time question word determines the tense of the sentence. While position is the primary structural rule, tense markers like đã and sẽ are also frequently used alongside khi nào for clarity:

• Future: Khi nào + (thì) + Subject + (sẽ) + Verb? (Placed at the beginning. Adding thì makes the question sound more natural. Note: If you use the future marker sẽ, khi nào can also naturally sit at the end of the sentence, e.g., Bạn sẽ đi khi nào?)
• Past: Subject + (đã) + Verb + khi nào? (Placed at the end)

Answering "khi nào" questions (The Mirroring Guideline): As a helpful pedagogical trick for beginners, when answering, you can simply replace khi nào with the specific time phrase, keeping it in the exact same position. If khi nào was at the beginning (future), your time phrase goes at the beginning. If it was at the end (past), your time phrase goes at the end. However, this is not a strict rule; specific time phrases carry their own tense and can often be placed at the end even for future answers (e.g., answering Khi nào bạn đi? with Tôi đi ngày mai).

Important Note on Tense and Dialects: While placing khi nào at the end for the past is standard Vietnamese grammar taught nationwide, many native speakers (especially in the South) prefer using dedicated past-tense words like hồi nào or lúc nào to clearly indicate the past without relying solely on word order. This is a highly common and valid usage.

Crucial Warning: Khi nào is primarily a question word or a conditional connector. Do not use it to mean 'when' in simple declarative statements (e.g., 'I was sleeping when he called'). For those statements, you must use khi or lúc.

Other uses and comparisons:

• Connecting clauses with thì: Khi nào connects clauses for future or conditional events, often paired with thì (e.g., Khi nào rảnh thì gọi cho tôi - When you are free, call me). Note that khi is a universal time connector used for past, present, general, and future statements (e.g., Khi tôi đến, tôi sẽ gọi - When I arrive, I will call).
• Always (Khi nào cũng): The structures Khi nào + Subject + cũng + Verb/Adjective and Subject + khi nào cũng + Verb/Adjective mean 'always' or 'whenever'. The latter is highly natural and common (e.g., Anh ấy khi nào cũng bận - He is always busy).
• Every time (Mỗi khi): mỗi khi is a core related connector meaning 'every time' or 'whenever' (e.g., Mỗi khi tôi buồn, tôi nghe nhạc - Whenever I am sad, I listen to music).
• Never (Không khi nào): Placing không before it creates không khi nào, meaning 'never' or 'rarely ever' (e.g., Tôi không khi nào đi trễ - I never arrive late).
• Indefinite pronoun: In statements, it means 'sometime' (e.g., Để khi nào tôi làm - I'll do it sometime).
• Vs. Bao giờ: bao giờ works exactly the same way, but is more colloquial and heavily preferred in Northern Vietnam.
• Vs. Chừng nào: Chừng nào is a highly common, interchangeable alternative to khi nào in Southern Vietnam, primarily used for the future or general 'when'.
• Vs. Lúc nào: Lúc nào is a highly common, interchangeable synonym for khi nào that can be used in both past and future contexts.