了 or 的: Past Events

Both 了 and 的 can talk about the past, but they answer different questions.

• 了 says that something happened or is finished: 他已经走了 (he has already left), 我昨天买了一本书 (I bought a book yesterday). Use it with 已经, with an amount (三杯, 一本), and to ask whether something happened: 你吃饭了吗？ (Have you eaten?)
• 是…的 is used when everyone already knows the event happened, and you focus on a detail: when, where, how or with whom. 你什么时候到的？ (When did you arrive?) 我是坐高铁来的 (I came by high-speed train).
• 是 is often left out, but 的 stays: at the end, or before the object: 你在哪儿学的中文？ (Where did you learn Chinese?)

Tip: a question word such as 什么时候, 在哪儿, 怎么 or 跟谁 about a past event almost always calls for 的, not 了.