The Mandarin 是…的 (shì...de) Construction: Highlighting When, Where, and How Something Happened

The principle: the action is given, the detail is news

是…的 is used when the fact that the action happened is already established or obvious, and what is actually new or being asked about is a CIRCUMSTANCE of it: when, where, how, with whom, by what means, by whom.

The pattern is:

Subject + 是 + time / place / manner / agent + verb + 的

The 是 sits right before the highlighted element. The 的 closes the clause. The verb lives between them.

我是昨天来的。 (Wǒ shì zuó tiān lái de.) - It was YESTERDAY that I came. (That I came is known; WHEN is the point.)
他是坐飞机来的。 (Tā shì zuò fēi jī lái de.) - He came BY PLANE. (That he came is known; HOW is the point.)
这本书是在北京买的。 (Zhè běn shū shì zài běi jīng mǎi de.) - This book was bought IN BEIJING. (WHERE is the point.)
这件事是他做的。 (Zhè jiàn shì shì tā zuò de.) - It was HE who did this. (WHO is the point.)

In speech 是 can be dropped in positive statements (他昨天来的), but it must stay in negatives and is safest to keep while learning.

What 是…的 can highlight

Any circumstance of the action. The element you want to stress goes straight after 是.

Time: 你是什么时候到的？ (Nǐ shì shén me shí hou dào de?) - WHEN did you arrive?
Place: 我们是在学校认识的。 (Wǒ men shì zài xué xiào rèn shi de.) - We met AT SCHOOL.
Manner / means: 我是走路去的。 (Wǒ shì zǒu lù qù de.) - I went ON FOOT.
Agent / who: 这张照片是我爸爸拍的。 (Zhè zhāng zhào piàn shì wǒ bà ba pāi de.) - This photo was taken BY MY DAD.
Purpose / for whom: 我是为了你才来的。 (Wǒ shì wèi le nǐ cái lái de.) - I came FOR YOUR SAKE.

The construction only works for actions that have ALREADY happened. You cannot use 是…的 for future or habitual actions - there is no settled past event to take apart.

The contrast with 了

This is the distinction that matters most, and the one exams test. Both involve a completed action, but they do different jobs.

了 REPORTS that something happened - it delivers the event as news:

我昨天去了北京。 (Wǒ zuó tiān qù le běi jīng.) - I went to Beijing yesterday. (Telling you the event.)

是…的 takes an event already known to have happened and HIGHLIGHTS a circumstance:

我是昨天去北京的。 (Wǒ shì zuó tiān qù běi jīng de.) - It was yesterday that I went to Beijing. (We both know I went; the point is WHEN.)

A worked contrast:

A asks: 你去过北京吗？ (Have you been to Beijing?) - B replies: 去过，我去了两次。 (Yes, I've been twice.) - reporting.
A asks: 你是怎么去的？ (How did you get there?) - B replies: 我是坐高铁去的。 (I went by high-speed rail.) - highlighting the means, because the fact of going is already on the table.

Rule of thumb: if the listener does not yet know the action happened, use 了 to report it. Once they know it happened and you are filling in or asking about a detail, switch to 是…的.

Negation: 不是…的 (never 没)

The negative of 是…的 is 不是…的. Crucially you do NOT use 没, even though the action is in the past, because you are not denying that it happened - you are denying a CIRCUMSTANCE.

我不是昨天来的，我是今天来的。 (Wǒ bú shì zuó tiān lái de, wǒ shì jīn tiān lái de.) - I didn't come yesterday, I came today.
这本书不是在北京买的。 (Zhè běn shū bú shì zài běi jīng mǎi de.) - This book wasn't bought in Beijing.
这件事不是我做的。 (Zhè jiàn shì bú shì wǒ zuò de.) - It wasn't me who did this.

Contrast with 没, which would deny the whole event: 我没来 (I didn't come at all). 不是…的 concedes the event and corrects the detail.

Questions

Question forms keep the 是…的 frame and slot the question word into the highlighted position. 是 stays in.

你是什么时候来的？ (Nǐ shì shén me shí hou lái de?) - When did you come?
你是在哪儿买的？ (Nǐ shì zài nǎr mǎi de?) - Where did you buy it?
你是怎么来的？ (Nǐ shì zěn me lái de?) - How did you come?
你是跟谁一起去的？ (Nǐ shì gēn shéi yì qǐ qù de?) - Who did you go with?
这是不是在网上买的？ (Zhè shì bu shì zài wǎng shang mǎi de?) - Was this bought online? (是不是 yes/no form.)

These are the natural way to ask about the circumstances of something you already know happened, which is why they are everywhere in real conversation.

Object placement

When the verb has an object, you have two acceptable positions for it, and a slight register difference between them.

的 at the very end, after the object: 我是在北京买这本书的。 (Wǒ shì zài běi jīng mǎi zhè běn shū de.)
的 right after the verb, object trailing: 我是在北京买的这本书。 (Wǒ shì zài běi jīng mǎi de zhè běn shū.)

Both are correct. The second pattern (的 hugging the verb) is very common in colloquial speech, especially with longer objects. If the object is a personal pronoun, keep 的 at the end: 我是昨天看见他的 (I saw him yesterday), not 我是昨天看见的他.

Worked examples

你是坐什么车来的？ - 我是坐地铁来的。 (Nǐ shì zuò shén me chē lái de? Wǒ shì zuò dì tiě lái de.) - What did you come by? - I came by underground. Manner highlighted, question and answer mirror each other.
这双鞋是在哪儿买的？ - 是在网上买的。 (Zhè shuāng xié shì zài nǎr mǎi de? Shì zài wǎng shang mǎi de.) - Where were these shoes bought? - Online. Place; subject dropped in the answer.
我不是故意的。 (Wǒ bú shì gù yì de.) - I didn't do it on purpose. Negated manner, a fixed everyday phrase.
他是去年大学毕业的。 (Tā shì qù nián dà xué bì yè de.) - He graduated from university LAST YEAR. Time highlighted.
这个决定是我们一起做的。 (Zhè ge jué dìng shì wǒ men yì qǐ zuò de.) - This decision was made by us TOGETHER. Agent / manner.
Compare: 我买了一台新电脑。 (Wǒ mǎi le yì tái xīn diàn nǎo.) - I bought a new computer. 了, reporting the event. vs 我是上个月买的。 (Wǒ shì shàng ge yuè mǎi de.) - I bought it last month. 是…的, highlighting when, the purchase already known.

Common mistakes

Using 没 to negate it. The negative is 不是…的, not 没…的. 没 denies the whole event; 是…的 corrects a detail of an event that did happen.
Dropping 是 in a negative. 是 is optional in positive statements but compulsory in negatives and questions of the 是不是 type. 我不是昨天来的, never 我不昨天来的.
Using it for actions that have not happened. 是…的 needs a settled past event. You cannot say 我是明天去的 for a future trip; use a plain future construction instead.
Confusing it with 了. 了 reports a new event; 是…的 spotlights a circumstance of a known one. If the listener does not yet know the action occurred, 是…的 is the wrong tool.
Putting a pronoun object after 的. With a pronoun object, 的 goes at the end: 我是昨天碰到他的, not 我是昨天碰到的他.
Forgetting that the verb sits between 是 and 的. The frame wraps the verb. 是…的 is not a tag you bolt on; the whole verb phrase lives inside it.