Preposition: 当 (When / At the Time of)
介词：当
Use 当 to set the time frame of an action, meaning "when" or "at the time of"
Place 当 at the beginning of a time clause, followed by the subject and verb, then 的时候 to close the time clause. The main clause follows.
当 is more formal and literary than just using 的时候 alone. In spoken Chinese, 的时候 by itself is often sufficient, but 当……的时候 adds emphasis and a slightly more polished tone.
Lesson Targets
Podcast
Podcast: Preposition: 当 (When / At the Time of) (介词：当)
Listen to Jason & Amy explain the 介词：当 pattern
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Understanding 介词：当
You already know 的时候 (when / at the time of) from HSK 1. Now you are upgrading with 当, which acts like a frame around the time clause. Think of 当……的时候 as a spotlight — it highlights the exact moment something happened. 当他进来的时候 means "at the very moment he came in." It is more precise and dramatic than simply saying 他进来的时候. You will see 当 in storytelling, news reports, and formal writing. In casual speech, many Chinese speakers drop 当 and just use 的时候, but understanding 当 is essential for reading and for sounding more articulate. The pattern is always the same: 当 + someone does something + 的时候, then the main event.
Key Points
- 当……的时候 (dāng…de shíhou) = "when / at the time that" — sets the time frame for the main clause.
- 当 adds emphasis and formality compared to using 的时候 alone.
- The 当 clause always comes BEFORE the main clause.
- 当 is often paired with 正在 to describe what was happening at that moment: 当他来的时候，我正在睡觉.
- In very formal or literary Chinese, 当 can appear without 的时候: 当此之际 (at this juncture).
- 当 can also pair with 已经: 当我到的时候，他已经走了 (When I arrived, he had already left).
- Do not confuse this 当 with 当 meaning "to serve as" — context makes it clear.
Chinese storytelling loves setting the scene with time expressions. Using 当……的时候 is a classic way to begin a narrative moment, similar to "Once upon a time, when…" in English fairy tales.
Key Vocabulary
Example Sentences
当他进来的时候，我们正在看电视。
When he came in, we were watching TV.
Sets the scene — what was happening at that moment
当爸爸回来的时候，妈妈已经做好晚饭了。
When Dad came back, Mom had already finished making dinner.
当老师走进教室的时候，大家都安静了。
When the teacher walked into the classroom, everyone became quiet.
当我听到这个消息的时候，非常高兴。
When I heard this news, I was very happy.
当下雨的时候，我们就在家看书。
When it rains, we read at home.
Habitual action
当你需要帮助的时候，可以给我打电话。
When you need help, you can call me.
当春天来的时候，花都开了。
When spring comes, all the flowers bloom.
当我还是学生的时候，每天都很忙。
When I was still a student, I was busy every day.
Common Mistakes
The 当……的时候 clause sets the scene and normally comes FIRST, before the main clause.
Do not forget 的 — the full phrase is 的时候, not just 时候.
Practice Exercises
Tips & Tricks
当……的时候 is your storytelling tool — use it whenever you want to set the scene before describing an event.
In casual speech, you can often drop 当 and just say 的时候: 他来的时候，我在睡觉. Adding 当 makes it more emphatic.
Practice combining 当 with 正在 (was doing) and 已经 (already) — these pairs appear constantly in Chinese.
When writing, 当……的时候 gives your sentences a polished, literary quality.
Homework
Write eight sentences about your daily life using 当……的时候. Include at least two with 正在 and two with 已经. Example: 当我起床的时候，太阳已经出来了。