是...的 Construction

When you already know that something happened and want to ask about or emphasize a particular detail — when it happened, where, how, or with whom — Chinese uses 是...的. Think of it as a spotlight: it shines light on one detail of a completed event.

Compare: 我去年去了中国 (I went to China last year) with 我是去年去的中国 (It was last year that I went to China). The first simply states the fact. The second emphasizes "last year" — perhaps answering the question "When did you go to China?"

The pattern is flexible. 是 goes before the detail you want to highlight, and 的 goes at the end. If the sentence has an object, 的 can go before or after it: 我是在北京学的中文 or 我是在北京学中文的 — both are correct, though placing 的 before the object sounds more natural in speech.

In casual conversation, 是 is often dropped: 你（是）什么时候来的？ — "When did you come?" But 的 is always required; without it, the emphasis disappears. This construction only works for completed actions — you cannot use it for future events.