是…的
shì…de
是…的 is a spotlight on something already done
The picture The event already stands on the stage. You switch on one spotlight and light up a single part of it: when, where, who, or how. 了 announces that something has come onto the stage; 是…的 lights one square of what is already there. 他来了 is the news that he came; 他是昨天来的 answers when.
Where the picture shows up in 是…的
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spotlight on the means
我是坐地铁来的。
wǒ shì zuò dìtiě lái de
I came by subway.
You are already there in the dark, and the spotlight picks out only the subway turnstile behind you.
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spotlight on the time
你是什么时候到的？
nǐ shì shénme shíhou dào de
What time did you get here?
The person on stage is left as they are and the light falls on a single wall clock.
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spotlight on the place
这件衣服是在网上买的。
zhè jiàn yīfu shì zài wǎng shàng mǎi de
I bought this online.
The clothes already hang on the rail, and the light picks out the order page on the laptop screen.
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spotlight on the year
他是去年来北京的。
tā shì qùnián lái Běijīng de
He came to Beijing last year.
He is standing on a Beijing street, and the beam lands only on one page of last year's calendar behind him.
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spotlight on the person
这是谁做的？
zhè shì shéi zuò de
Who made this?
The finished thing is left where it is and the light falls only on the hands that made it.
To announce that something happened, use 了. Use 是…的 when everyone already knows it happened and only the how is in question.
All of it in one picture
Light on the turnstile asks by what means; light on the wall clock asks at what hour. Light on the order page asks where, light on last year's calendar asks when, light on the hands asks who. All five times the event is already standing on the stage; the only thing that moves is the beam. 是…的 is not an announcement, it is the switch that lights up one square of something everyone already knows happened.