In the perfect tense, with verbs ending in -er, how do you know when to write "é/és/ée/ées" at the end of the verb?

In the perfect tense, with most verbs, you will use:-> a pronoun + the verb "avoir" in the present tense + the past participle of the verb you're usingFor instance: "J'ai mangé une pomme."For transitive verbs, like the verb "manger", the object can be placed in front of the verb, for instance:"Je l'ai mangée."Here, because we're talking about the apple, which is a feminine noun in french, you need to make the verb agree with the object (the apple), therefore manger is conjugated as "mangée" (because an apple is a singular feminine noun).If we were talking about "apples" in the sentence, then we would add "ées" at the end of the verb because it is a plural and feminine noun -> "Je les ai mangées."