什么时候 vs 多久: asking ‘when’ vs ‘how long’
什么时候 (shénme shíhou) asks about a specific point in time when an event happens (e.g., “When did you arrive?”). 多久 (duōjiǔ) asks about duration—how long an action or state lasts (e.g., “How long have you been studying?”). Choosing the right one depends on whether you want to know a timepoint or a length of time.
The core difference is that 什么时候 targets a specific moment or range (e.g., “at what time/day/year”), while 多久 targets the length of time from start to end. For past events, 什么时候 uses the 是...的 construction (e.g., 你什么时候来的？) to refer to a past timepoint—do not simply add 了. For future events, 什么时候 can be used with 会 or 要. 多久 often pairs with 了 to ask how long something has been ongoing, or with 要 for future duration. Both can be used in the same sentence pattern but answer different types of information.
When to use each
Use to ask for a specific point in time of an event—past, present, or future. For past events, the verb is placed in the 是...的 structure (e.g., 他什么时候来的？). For future, it can be used with 会 or 要 (e.g., 你什么时候来？). It cannot be used to ask about duration.
In casual speech, 什么时候 can also be used as a relative clause (e.g., 我什么时候说过了？ 'When did I say that?') to express disbelief.
Use to ask about the length of time an action or state lasts. Often combined with 了 to ask about a completed period up to now (e.g., 你学了多久了？ 'How long have you studied?'). Also used with 要 for future duration (e.g., 你要待多久？ 'How long will you stay?'). It cannot be used to ask for a specific timepoint.
In formal or written Chinese, 多长时间 (duō cháng shíjiān) can replace 多久, but 多久 is more colloquial and common in questions.
At a glance
| 什么时候 | 多久 | |
|---|---|---|
| Question type | Asks for a timepoint (when) | Asks for duration (how long) |
| Expected answer | A specific time (e.g., 昨天下午, 明年) | A length of time (e.g., 三年, 两个小时) |
| Use with 的 for past | Requires 是...的 for past specific time (e.g., 你什么时候来的？) | Does not use 的 for past duration; use 了 (e.g., 你学了多久？) |
| Use with 了 | Rarely used with 了 to mark a past timepoint (wrong: 你什么时候来了？) | Commonly used with 了 to indicate 'up to now' (e.g., 你来了多久了？) |
| Future reference | Can be used with 会/要 (e.g., 你什么时候会来？) | Can be used with 要 (e.g., 你要去多久？) |
Examples
- 什么时候你什么时候来的？nǐ shénme shíhou lái de?When did you come?Uses 的 in the 是...的 construction to ask about a past timepoint.
- 什么时候他什么时候到？tā shénme shíhou dào?When will he arrive?Future timepoint question, no 的 needed.
- 多久你学中文多久了？nǐ xué Zhōngwén duōjiǔ le?How long have you been studying Chinese?Uses 了 to mark duration up to now.
- 多久你要在北京待多久？nǐ yào zài Běijīng dāi duōjiǔ?How long will you stay in Beijing?Future duration question with 要.
- 什么时候你什么时候去了北京？nǐ shénme shíhou qù le Běijīng?When did you go to Beijing? (incorrect)✗ Wrong: uses 了 instead of 的. Correct: 你什么时候去的北京？
- 多久你多长时間没睡觉了？nǐ duō cháng shíjiān méi shuìjiào le?How long has it been since you slept?Shows 多长时间 as a formal alternative to 多久.
Common mistakes
- Using 多久 to ask for a point in time (e.g., 你多久来？) — correct: 你什么时候来？
- Adding 了 after the verb with 什么时候 for past events (e.g., 你什么时候来了？) — correct: 你什么时候来的？ (use 的)
- Using 什么时候 to ask about duration (e.g., 你什么时候学？ with expectation of 'two years') — correct: 你学多久了？
- Omitting 了 after 多久 when asking about a duration that continues to the present (e.g., 你学中文多久？ should be 你学中文多久了？)
FAQ
- When do I use 什么时候 vs 多久?
- Use 什么时候 to ask about a specific time (e.g., “When is the meeting?”). Use 多久 to ask about the length of time (e.g., “How long is the meeting?”). Think 'timepoint' vs 'duration'.
- Can I use 多久 to ask when something starts or ends?
- No. 多久 only asks for the length of time between start and end. If you want the actual start or end time (e.g., “When does the class start?”), use 什么时候.
- Why do I sometimes see 了 with 什么时候, like in 你什么时候来了？
- That form is generally incorrect for asking about a past timepoint. The correct pattern for a past event is 你什么时候来的？ (是...的 construction). However, 你什么时候来了？ can occur in very specific contexts (e.g., expressing surprise: “When did you come?!”), but it is not the standard way to ask for a time.