1. Explain it simply
了 right after a verb marks that the action happened or is finished.
To make it negative, don't add 没(有) AND 了 together - drop the 了 and use 没(有) + verb instead.
2. Show examples
我吃了早饭。
Wǒ chī le zǎofàn.
I ate breakfast.
Verb + 了 + object
他买了一本书。
Tā mǎi le yī běn shū.
Sandhi:Tā mǎi le yì běn shū.
He bought a book.
了 shows the purchase is complete
我没吃早饭。
Wǒ méi chī zǎofàn.
I didn't eat breakfast.
Negative: 没 + verb, no 了
3. Share a tip
Never say 没吃了 - 没(有) and completed-action 了 don't appear together. Drop the 了 when you negate with 没.