是…的【shì … de】

是…的 is for an event both speakers already know happened. It picks out one detail of that event – the time, the place, the means, the company, the doer – and puts that detail right after 是.

Whatever sits right after 是 is the focus. Move 是 and the focus moves with it.

Who did it

Put the doer after 是 and the thing done before it.

News or detail

To tell someone something happened, use 了. Once it's known, ask or answer about the details with 是…的.

No 了 inside

是…的 already marks the event as done, so 了 doesn't go in it.

• × 我是昨天来了 → 我是昨天来的
• × 我是昨天来了的 → 我是昨天来的

Negation and dropping 是

The negative is 不是…的; 是 can't drop there. In a positive sentence 是 often drops in speech, and 的 stays.

• × 我不昨天来的 → 我不是昨天来的

Where the object goes

An object can go before or after 的: 我是昨天买的票 and 我是昨天买票的 both work. With a pronoun object, 的 goes last.

Purpose

是 + 来 / 去 + verb phrase + 的 says what someone came or went to do.

是…的 can also wrap a statement to sound firm and certain – 他会来的, 这样是不行的. That use has nothing to do with past events.