"My cheese pizza, I already ate it."
Translation:Ma pizza au fromage, je l'ai déjà mangée.
Thank you Jenny, looking deeper, I can see that mangée does not appear nowhere in Passé composé: https://leconjugueur.lefigaro.fr/conjugaison/verbe/manger.html
However, it appears only as participe pase.
I must say I am still puzzled...
So you know how some verbs (aller, partir, monter, devenir, etc) use être when conjugated in the passé composé? Remember how the ending of the past participles then agrees with the subject of the sentence? E.g. elle est devenue.
We normally learn that when a verb uses avoir to conjugate in the passé composé, you don't have to worry about the past participle agreeing with anything. But, of course, there is an exception.
When the direct object is placed prior to the verb, the past participle agrees with the direct object in gender and number. E.g. la pizza qu'il a mangée (mangée agrees with the feminine singular pizza).
The things to keep in mind:
you must be using the passé composé
the verb must use avoir to form the passé composé
the direct object must come before the verb
If all these things are true, the past participle agreees with the direct object (not the subject) in gender and number).
Here is a link that might help: https://www.thoughtco.com/french-agreement-compound-verbs-works-4086482