How to Say “When” with 当 (dāng)
当……的时候 (dāng … de shíhou) means “when,” framing a time clause at the front of the sentence: 当 opens the bracket, 时候 (spoken) or 时 (formal) closes it: 当我年轻的时候，学费很便宜 (“when I was young, tuition was cheap”). The “when” itself is really 的时候; 当 is an optional, slightly formal opener you can always drop (我年轻的时候 means the same). But 当 needs a full event-clause after it, the time clause comes first, and 当 can never ask “when?” (that's 什么时候).
Why this trips learners up
The everyday way to say “when” in Chinese is ……的时候, tacked onto the end of a time clause (我走的时候 = “when I leave”). 当 (dāng) is what you add at the front to bracket that clause more explicitly: 当 + event-clause (+ 的) + 时候，main clause. 当 opens the frame and 时候 closes it, wrapping the time clause like a pair of brackets: 当我还是学生的时候，学费很便宜 (“when I was a student, tuition was cheap”).
Here's the key thing, and the source's own point: 当 is optional. The “when” work is done by 时候, not by 当, so you can drop 当 and the sentence means exactly the same: 我年轻的时候 = 当我年轻的时候. What 当 buys you is a slightly more formal, emphatic, signposted feel: it announces “at the moment that…” up front, which is especially handy when the time clause is long (当大家都不看好他的时候，他反而成功了). The closer comes in two flavours: 时候 is the neutral, spoken one; 时 is terser and more literary (当飞机起飞时，请系好安全带). With a short clause you can even drop the closer and lean on 当 alone: 当我打开门，一只猫跑了进来.
Now the mistake almost every learner makes at least once: using 当 to ask “when.” 当 only ever introduces the time frame of a statement; it can never form a question. “When are you free?” is 你什么时候有空？, never 当你什么时候有空？. Chinese keeps the two jobs in separate words: 当…(时候) for “when” as a conjunction (“when X happens, …”), and 什么时候 for “when?” as a question word. If you're asking, it's 什么时候; if you're setting a scene, it's 当…时候.
Two more guardrails. First, 当 needs a full event-clause after it, something with a verb, not a bare time word: 当明天 is wrong (明天 is already “tomorrow”), you'd just say 明天我们去爬山. 当 brackets “when something happens,” so there has to be a happening inside it. Second, the time clause comes first: 当我上台的时候，我很紧张 (“when I go on stage, I get nervous”), not 我很紧张当我上台的时候 the way English can trail “when” onto the end. And for a habitual “whenever,” front it with 每当: 每当我听到这首歌，就会想起大学生活 (“whenever I hear this song, I think of college”).
The structure
Colour key
Each colour marks one grammatical role — and the same colour means the same role on every page in the Lab.
Examples in context
Real-world sentences, easiest first. Toggle pinyin or the translation, tap any word to see its role, or play the audio.
Tap a word to see its grammatical role.
dāng 当 Pattern wǒ 我 Subject hái shì 还是 Verb xuéshēng 学生 Object de 的 Function word shíhou 时候 Pattern xuéfèi ，学费 Subject hěn 很 Adverb piányi 便宜 Adjective
When I was still a student, tuition was cheap (basic 当…的时候).
dāng 当 Pattern háizi 孩子 Subject kū 哭 Verb de 的 Function word shíhou 时候 Pattern māma ，妈妈 Subject jiù 就 Adverb huì 会 Function word bào 抱 Verb tā 他 Object
When the child cries, mom picks him up (当…的时候 with 就 in the main clause).
dāng 当 Pattern fēijī 飞机 Subject qǐfēi 起飞 Verb shí 时 Pattern qǐng ，请 Adverb jì hǎo 系好 Verb ānquándài 安全带 Object
When the plane takes off, please fasten your seatbelt (当…时, the formal closer).
dāng 当 Pattern wǒ 我 Subject dǎkāi 打开 Verb mén 门 Object yì zhī ，一只 Measure word māo 猫 Subject pǎo le jìnlái 跑了进来 Verb
When I opened the door, a cat ran in (当 with the closer dropped before a short clause).
měi dāng 每当 Pattern wǒ 我 Subject tīng dào 听到 Verb zhè shǒu gē 这首歌 Object jiù ，就 Adverb huì 会 Function word xiǎng qǐ 想起 Verb dàxué shēnghuó 大学生活 Object
Whenever I hear this song, I think of my college days (每当 = “whenever”).
dāng 当 Pattern dàjiā 大家 Subject dōu 都 Adverb bù 不 Negation kànhǎo 看好 Verb tā 他 Object de 的 Function word shíhou 时候 Pattern tā ，他 Subject fǎn’ér 反而 Adverb chénggōng 成功 Verb le 了 Function word
When nobody was optimistic about him, he succeeded instead (a long time clause, where 当 earns its keep).
dāng 当 Pattern wǒ 我 Subject yìshí dào 意识到 Verb wèntí de 问题的 Other yánzhòngxìng 严重性 Object shí 时 Pattern yǐjīng ，已经 Adverb tài wǎn 太晚 Adjective le 了 Function word
By the time I realized how serious the problem was, it was already too late (当…时).
dāng 当 Pattern zhěnggè shìjiè 整个世界 Subject dōu 都 Adverb zài 在 Function word huáiyí 怀疑 Verb nǐ 你 Object shí 时 Pattern gèng ，更 Adverb yào 要 Function word xiāngxìn 相信 Verb zìjǐ 自己 Object
When the whole world doubts you, you must believe in yourself all the more (formal / emphatic 当…时).
Common mistakes
Why it happens: 当 introduces the time frame of a statement; it can never ask “when?”. “When are you free?” is 你什么时候有空？, using the question word 什么时候, not 当. Keep the two apart: 当…(时候) sets a scene (“when X happens”); 什么时候 asks a question (“when?”).
Why it happens: 当 needs a full event-clause (a verb) after it, not a bare time word. 当明天 is wrong because 明天 is already “tomorrow”; just say 明天我们去爬山. 当 brackets “when something happens,” so there must be a happening inside it (当明天出发的时候… would need the verb 出发).
Why it happens: The 当…时候 clause comes first, then the main clause. English can trail “when” onto the end (“I get nervous when I go on stage”), but Chinese fronts it: 当我上台的时候，我很紧张. Put the time frame first, the main statement second.
Compare & contrast
| 当……的时候 (with 当: a fronted, slightly formal “when”) | ……的时候 (plain “when,” 当 dropped: same meaning) | The difference |
|---|---|---|
| 当我年轻的时候dāng wǒ niánqīng de shíhou = “when I was young” (with 当, a touch formal) | 我年轻的时候wǒ niánqīng de shíhou = “when I was young” (plain, everyday; 当 dropped, same meaning) | 当我年轻的时候 and 我年轻的时候 both mean “when I was young.” The “when” is carried by 的时候 either way; 当 just adds a fronted, slightly more formal, signposting feel. Drop it and nothing essential changes. |
| 当他说这话的时候dāng tā shuō zhè huà de shíhou = “when he said this” (with 当) | 他说这话的时候tā shuō zhè huà de shíhou = “when he said this” (without 当; identical meaning) | 当他说这话的时候 and 他说这话的时候 both mean “when he said this.” Again the 当 is optional; it brackets the clause a touch more explicitly, but 的时候 alone already says “when.” Use 当 to signpost a long or emphatic time clause; skip it in everyday speech. |
Try it yourself
Say “When I got home, it was already dark.” Front the time clause with 当…的时候.
Related patterns
Quick reference card
How to Say “When” with 当 (dāng) grammar.merrymandarin.com A pocket summary — print it and keep it by your desk.