了 for completed actions; negation with 没 (有)

Right after a verb, 了 marks the action as completed: 他买了一本书 (he bought a book).

With a simple object (no number), 了 usually goes at the end: 我吃饭了 / 他吃早饭了.

Negative: use 没 or 没有 before the verb, and drop 了: 他没买书 (he didn't buy a book). 我今天没有吃早饭.

Question: 你吃饭了吗？ or 你吃没吃饭？ / 你吃饭了没有？ Common negative answer: 还没（呢） (not yet).

Careful: 了 isn't a past tense. Past habits don't take it (“I used to go often”), and 了 can even be used for the future (“once I've eaten…”).