khi

Khi attaches directly to a subject and verb, or just a verb phrase if the subject is understood. It can also attach directly to adjectives (stative verbs) to express states of being (e.g., khi rảnh, khi buồn), or to nouns and noun phrases indicating a period of life, often with còn or là (e.g., khi còn sinh viên or khi còn là học sinh). Note that to talk about past periods of life, hồi is frequently used instead of khi (e.g., hồi còn nhỏ). Because Vietnamese is a tense-neutral language, the timeframe of a khi clause is established by context or time markers like đã (past) or sẽ (future).

Khi can sit at the beginning of a sentence, or in the middle to connect two clauses without a comma. It also acts as an essential building block for compound time conjunctions like trước khi (before), sau khi (after), trong khi (while), and cho đến khi (until). Technically, khi is a noun meaning "time" or "moment", which is why it forms the basis for frequency adverbs like có khi, đôi khi (sometimes), or ít khi (rarely).

Key differences and nuances to remember:

• Khi vs. Lúc: Both mean "when" and are highly interchangeable. However, khi is generally slightly more formal or literary and refers to broader timeframes, whereas lúc is more conversational and tends to pinpoint a specific, exact moment. Crucially, khi requires an action, event, or state (a verb or adjective phrase) and cannot attach directly to a bare clock time. You must use lúc as the preposition "at" for clock times (lúc 3 giờ), or add a verb for khi (khi đến 3 giờ).
• Statements vs. Questions: By itself, khi is only used for statements. To ask "when?", you must use question words like khi nào, bao giờ, lúc nào, chừng nào (a common Southern equivalent for future or general time), or hồi nào (a specific Southern question word for "when" in the past). For these question words, position sets the tense as a general guideline: placing the question word at the beginning indicates the future (Khi nào bạn đi?), while placing it at the end indicates the past (Bạn đi khi nào? or Bạn đi hồi nào?). Note that in casual Southern Vietnamese, there is flexibility where chừng nào (and sometimes khi nào) can be placed at the end of a sentence and still refer to the future. For habitual or routine questions, the question word is typically placed at the end along with a frequency adverb like thường or hay (e.g., Bạn thường ăn sáng khi nào?). Explicit tense markers can override this position rule entirely.