是…的【shì…de】

是…的 is for an event both speakers already know has happened. It moves the focus onto one detail of it – when, where, how, or who with. 是 goes before that detail and 的 at the end of the sentence.

The question takes for granted that you came; it asks only when.

了 or 是…的

了 reports that something happened. Once that's established, the questions and answers about the details switch to 是…的.

Learners keep 了 for the follow-up, or leave off 的.

• × 你什么时候来了？ → 你是什么时候来的？
• × 我是昨天来了 → 我是昨天来的

Dropping 是

In speech 是 is often left out. The 的 stays.

In the negative, 不是 has to be said.

With an object

When the verb has an object, 的 can go before it or at the end. Both are common.

是…的 has other jobs too, such as stating a view firmly. Those come later, in their own point.