bao giờ / khi nào

Bao giờ and khi nào are standard words used to ask about a general timeframe (a day, month, year, etc.). While tense is ultimately determined by context or time words, there is a very strong default pattern based on position:

• Front of the sentence: Usually asks about the future ("When will you...?")
• End of the sentence: Usually asks about the past ("When did you...?")

How to answer:
To answer these questions, simply replace the time word (bao giờ or khi nào) with the actual time phrase, keeping it in the exact same position. If the question word was at the front (future), put your time phrase at the front. If it was at the end (past), put your time phrase at the end.

Differences and Regional Usage:
While bao giờ is heavily Northern, khi nào is universally used across all regions, including the South, as the most common way to ask "when". In Southern Vietnamese, you will also frequently hear chừng nào or hồi nào. Chừng nào is used for future or present questions (and while usually at the front, its position is slightly more flexible in casual Southern speech and can appear at the end of a sentence, e.g., Em về chừng nào?). Hồi nào is used at the end of a sentence for past questions. Hồi nào is also heavily used in casual Southern speech as a rhetorical expression of denial meaning "Since when?" or "I never did that!" (e.g., Tôi nói vậy hồi nào?).

Lúc nào:
While bao giờ and khi nào generally follow the positional tense rule, lúc nào literally translates to "which moment" and bridges the gap between "when" and "what time". Its position is more flexible and relies more on context or tense markers rather than strict placement. It is also commonly used in the pattern lúc nào cũng to mean "always" or "at all times" (e.g., Anh ấy lúc nào cũng bận - He is always busy). Note that bao giờ cũng and khi nào cũng are also widely used to mean "always" following the exact same structure.

Anytime is fine:
A highly useful pattern is adding cũng được after these time words. For example, Khi nào cũng được or Lúc nào cũng được is the standard way to say "Anytime is fine".

Never vs. Have Never:
In statements, these words can mean "ever" or "never". It is crucial to distinguish between không bao giờ (never, as a general habit or absolute fact) and chưa bao giờ (have never, meaning not yet up to this point in your life). Note that while không bao giờ and chưa bao giờ are the most common, không khi nào and chưa khi nào also exist and mean the same thing. To ask "Have you ever...?", you can use the structure đã bao giờ ... chưa? (though Southern speakers heavily prefer the structure có ... lần nào chưa? or có bao giờ ... chưa?). In spoken Vietnamese, especially in the South, it is very common to replace đã with có at the beginning of the sentence when asking "Have you ever...?" (e.g., Có bao giờ anh ăn phở chưa?). Remember that questions ending in chưa should be answered with rồi (yes/already) or chưa (no/not yet), not có or không. To answer affirmatively that you have done something, you can say đã từng or simply rồi. To give a full negative answer to a question like Có bao giờ... chưa?, chưa bao giờ is the standard response.

Don't confuse "when" with "what time" or "how long":
If you want to ask for a specific clock time, use mấy giờ instead. If you want to ask about a duration of time ("how long"), use bao lâu.

Question vs. Statement:
Be careful not to confuse the question words khi nào and bao giờ (when?) with the statement connector khi (when / at the time that). A common learner mistake is using khi nào or bao giờ to mean "when" in a past narrative statement (e.g., "When I was young..."). You must use khi or lúc for this (e.g., Khi tôi còn trẻ...), not Khi nào tôi còn trẻ.... While khi is the standard statement connector for "when", it cannot be used as a standalone question word (e.g., you cannot say Bạn đi khi?). Khi nào can also be used in statements to mean "whenever" (e.g., Khi nào rảnh thì gọi tôi).

A note on tense markers:
Vietnamese relies heavily on context and time words rather than strict tense markers. Therefore, adding đã (past) or sẽ (future) is optional. However, native speakers still frequently use them for clarity or rhythmic flow, so sentences like Khi nào bạn sẽ đi? are perfectly natural.