"I have not heard it."

Translation: Je ne l'ai pas entendu.

[Daleswords]

je ne l'ai entendu. Isn't pas optional in casual spoken French? Or did I do something else wrong here?

[relox84]

It's actually the opposite: the 'ne' is ommitted in casual spoken French
(Je ne l'ai pas entendu -> Je l'ai pas entendu), but this ommission is not standard and thus usually not accepted on duolingo.

[saeedsoltaniiran]

Why we used entendue here instead of entendu

[Ehsan_Mehmed]

It depends. In this case, "it" in this sentence refers to a feminine word

[rstrobach]

What is the difference between ecouter and entendre?