The negative form of le sentences:

Q：Nǐ chī le zǎofàn mɑ？ Q：你 吃 了早 饭 吗？ Have you had your breakfast? A：Méichī。 A：没 吃 。 No. (Haven’t eaten) Q：Nǐ chī zǎofàn mɑ？ Q：你 吃 早 饭 吗？ Do you have your breakfast? A：Wǒ bù chī zǎofàn。Wǒ měitiān zǎoshɑnɡ zhǐ hē kāfēi。 A：我 不 吃 早 饭 。我 每 天 早 上 只 喝 咖啡 。 I don’t have breakfast. I only drink coffee every morning. Q：Tā shòu le mɑ？ Q：她 瘦 了吗？ Has she lost her weight? A：Méishòu。 A：没 瘦 。 No. Q：Tā shòu mɑ？ Q：她 瘦 吗？ Is she thin? A：Búshòu。 A：不瘦 。 No. (She is not thin.)

Since le comes after a predicate to show the action is finished or the situation changed. Its negative form should use mei and in the negative form le has to be omitted.