A2
是…的 — it was “then”, it was “there”
How the 是…的 pattern in Chinese puts the spotlight on when, where or how something happened, and why 我昨天來了 and 我是昨天來的 are different sentences.
是…的 is used when both of you already know the event happened, and the question is one of its details — when, where, how, with whom.
你是什麼時候來的？
Nǐ shì shénme shíhou lái de?
When did you get here?
我是昨天來的。
Wǒ shì zuótiān lái de.
I got here yesterday.
你是坐飛機來的嗎？
Nǐ shì zuò fēijī lái de ma?
Did you fly here?
The 是 goes in front of the detail being asked about, and 的 comes at the end. Everything between them is the part under the spotlight.
Against 了
Both sentences below are correct, and they are not interchangeable.
我昨天來了。
Wǒ zuótiān láile.
I came yesterday.
我是昨天來的。
Wǒ shì zuótiān lái de.
It was yesterday that I came.
The first reports that the coming happened — it is news. The second assumes you already knew I was here and tells you which day. If someone can see you standing in front of them, asking 你什麼時候來了 is odd; 你是什麼時候來的 is the natural question.
是 can be dropped, 的 cannot
In a positive statement the 是 is often left out and nothing changes.
我昨天來的。
Wǒ zuótiān lái de.
I came yesterday.
The 的 is what carries the pattern. Drop it and you have an ordinary sentence with different work to do.
Where the object goes
With an object, 的 usually comes before it rather than at the very end — though both are heard and neither will confuse anyone.
我是在北京學的中文。
Wǒ shì zài Běijīng xué de Zhōngwén.
I learned my Chinese in Beijing.
The negative keeps 是
不是…的, and this is the one place 是 cannot be dropped.
我不是昨天來的。
Wǒ bú shì zuótiān lái de.
It wasn’t yesterday that I came.
Note it is 不是 rather than 沒 — you are not denying that the coming happened, only correcting a detail of it. Which is the whole point of the pattern, visible in one word.