当……时
Reach for 当……时 when you want to set a precise time-frame for an event — 'when X happened, Y was happening too.' It's the literary version of plain 的时候: same meaning, more polish. You see it constantly in news leads and narrative writing because it lets the writer freeze a moment ('when the plane touched down at Beijing Capital Airport…') before delivering the next line. In speech, people skip the 当 and just say 时候.
Structure
当 [EVENT/TIME] 时, [MAIN CLAUSE]
dāng... shí...
How to Think About It
当 and 时 are bookends, not a single word. 当 opens the time-frame and 时 closes it — everything between them is the moment being framed. Think of it as cinematic framing: 当 says 'open the shot,' 时 says 'and now the camera cuts to what happens.' The main clause always comes after 时, never before, because the time-frame has to be set up first.
Examples
当我走进教室时，老师已经在讲课了。
Dāng wǒ zǒu jìn jiàoshì shí, lǎoshī yǐjīng zài jiǎngkè le.
When I walked into the classroom, the teacher was already lecturing.
当夜幕降临时，城市亮起了灯光。
Dāng yèmù jiànglín shí, chéngshì liàng qǐ le dēngguāng.
When night fell, the city lit up.
当他听到这个消息时，眼泪流了下来。
Dāng tā tīng dào zhège xiāoxi shí, yǎnlèi liú le xiàlái.
When he heard the news, tears rolled down his face.
Common Mistake
Learners forget the closing 时 because English 'when' is one word. 当……的时候 also works, but 当 alone without 时 or 的时候 dangles — the time-frame never closes.
当他到家，已经很晚了。
当他到家时，已经很晚了。
Don't Confuse With
……的时候
Colloquial equivalent. 当……时 belongs to news and essays; 的时候 belongs to conversation.
一……就……
Use 一……就…… for 'as soon as A, B' — tight sequencing. 当……时 just sets a backdrop without implying immediacy.
在……时
在……时 emphasizes the duration of an ongoing situation; 当……时 zooms in on a specific moment when something happened.
Practice
____我醒来时，天已经亮了。
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当
当电话响起____，他正在洗澡。
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时
Arrange: 时 / 当 / 下雨 / 开始 / 我们 / 出门了
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当开始下雨时，我们出门了。
Translate: When the news broke, everyone fell silent.
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当消息传出时，所有人都安静了下来。
Write one sentence using 当……时 to describe a memorable moment.
Show answer
当我第一次踏上长城时，我感到非常震撼。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: