English says "again" whether the repeat has already landed ("he's late again") or hasn't yet ("say it again"). Chinese won't let you be that vague: it forces you to declare whether the repetition is realized or not. 又 is "again" for a recurrence that has already happened — you're looking back at a repeat that's now a fact. 再 is "again" for a recurrence still to come — a plan, a request, a command about a repeat that hasn't occurred yet. The whole decision rides on one question: has it happened yet? Happened → 又; not yet → 再. Because 又 reports something realized, it almost always pairs with the perfective 了; because 再 points at the unrealized future, it doesn't. This page is the decision tool; for each adverb in full, see 又 for past repetition and 再 for future repetition.
The decision test: has the repeat happened yet?
| The repetition is… | Adverb | Typical frame | Example |
|---|---|---|---|
| already realized (past / just now) | 又 (yòu) | 又 … 了 | 他又迟到了 (late again) |
| not yet realized (future / request) | 再 (zài) | 再 … (no 了) | 明天再说 (talk again tomorrow) |
又: the repeat already happened
又 reports a recurrence that is now a fact — it happened again, and here we are noting it. It leans on the perfective 了 to seal that "it's done" reading.
他今天又迟到了。
tā jīntiān yòu chídào le
He was late again today. (informal) — a realized repeat, so 又…了.
糟糕，我又忘带钥匙了。
zāogāo, wǒ yòu wàng dài yàoshi le
Ugh, I forgot my keys again. (informal) — it's already happened: 又.
昨天又下了一场大雨。
zuótiān yòu xià le yì cháng dà yǔ
It rained heavily again yesterday. (neutral) — past recurrence, so 又.
再: the repeat is still to come
再 points at a recurrence that hasn't happened yet — a plan, a polite request, a command, an offer of "one more." No 了, because there's nothing realized to mark.
这事明天再说吧，今天太晚了。
zhè shì míngtiān zài shuō ba, jīntiān tài wǎn le
Let's talk about this again tomorrow, it's too late today. (informal) — a future repeat, so 再.
太好吃了，老板，再来一份！
tài hǎochī le, lǎobǎn, zài lái yí fèn!
So good — boss, one more portion! (informal) — requesting a repeat: 再.
没听清，你能再说一遍吗？
méi tīng qīng, nǐ néng zài shuō yí biàn ma?
I didn't catch that — could you say it once more? (informal) — an as-yet-unrealized repeat: 再.
The mirror pair
Set the same verb in both frames and the split is obvious. Only the time-status of the repeat — past vs future — changes, and with it the adverb and the 了.
他刚才又问了同样的问题。
tā gāngcái yòu wèn le tóngyàng de wèntí
He just asked the same question again a moment ago. (neutral) — already asked: 又…了.
如果还不懂，你可以再问我。
rúguǒ hái bù dǒng, nǐ kěyǐ zài wèn wǒ
If you still don't get it, you can ask me again. (neutral) — a future asking: 再.
The exception: certain, inevitable future recurrence
There's one honest wrinkle. 又 can reach into the future when the recurrence is treated as inevitable or already looming — a cyclical certainty, or something on the very brink of happening. These are set frames where the repeat feels as good as guaranteed, so Chinese lets 又 (often with 要…了) point forward. This is idiomatic, not a loophole to exploit — for an ordinary plan, still use 再.
唉，又要下雨了，衣服还没收。
āi, yòu yào xià yǔ le, yīfu hái méi shōu
Ugh, it's about to rain again, and the laundry's still out. (informal) — imminent, inevitable recurrence: 又要…了.
别灰心，明天又是新的一天。
bié huīxīn, míngtiān yòu shì xīn de yì tiān
Don't lose heart — tomorrow is another new day. (neutral) — a cyclical certainty, so 又.
Common Mistakes
1. Using 再 for a repetition that already happened. This is the top slip. A past recurrence is 又, with 了.
❌ 他昨天再迟到了。
Wrong — a realized past repeat takes 又: 他昨天又迟到了.
✅ 他昨天又迟到了。
tā zuótiān yòu chídào le
He was late again yesterday. — 又…了 for the realized repeat.
2. Using 又 for a request or command. Asking someone to do something again is unrealized, so it's 再.
❌ 请又说一遍。
Wrong — a request is future/unrealized: 请再说一遍.
✅ 请再说一遍。
qǐng zài shuō yí biàn
Please say it once more. — 再 for the unrealized repeat.
3. Dropping 了 with a realized 又. A past 又 recurrence normally needs its 了 to sound complete.
❌ 我又忘。
Incomplete — a realized repeat wants 了: 我又忘了.
✅ 我又忘了。
wǒ yòu wàng le
I forgot again. — 又…了.
4. Using 又 for an ordinary future plan. A repeat you intend to do (but haven't) is 再, not 又.
❌ 我下周又来看你。
Wrong for a plan — an intended future repeat is 再: 我下周再来看你.
✅ 我下周再来看你。
wǒ xià zhōu zài lái kàn nǐ
I'll come to see you again next week. — 再 for the planned repeat.
Key Takeaways
- 又 = "again" for a realized recurrence — it already happened (他又迟到了). Pairs with the perfective 了.
- 再 = "again" for an unrealized recurrence — plan, request, or command still to come (明天再说). Takes no 了.
- The whole decision: has the repeat happened yet? Yes → 又; not yet → 再.
- The one exception: 又 can point forward for inevitable or imminent recurrence in set frames (又要下雨了；明天又是新的一天).
- Don't confuse the "again" 又 here with the paired 又…又… construction ("both… and…"), which is a different job entirely.
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- The Perfective 了 (le₁): Completed ActionHSK 1 — The verb-suffix 了 (le₁) sits right after the verb and marks the action as completed or realized — 我吃了饭, 买了三本书. It is completion aspect, not past tense, so it can even point to the future: 下了课就走.