Ciao Luz,
Apologies for the mix-up. Let me explain.
In the lesson on the impersonal SI, the example given is: "Cosa fare se si è dimenticato di pagare?"
However, you are correct that the correct expression should be: "Cosa fare se ci si è dimenticati di pagare?"
In Italian, when using reflexive verbs in the impersonal form with the particle "si," the reflexive pronoun "si" is combined with the appropriate indirect object pronoun, which in this case is "ci." The use of "ci si" in this context means "one" or "people".
Regarding the participle agreement, it is generally correct that the past participle agrees with the direct object pronoun when using reflexive verbs. In this case, since the indirect object pronoun "ci" is combined with the reflexive pronoun "si," the participle "dimenticato" should agree with the plural "ci" and become "dimenticati."
So, the correct sentence is: "Cosa fare se ci si è dimenticati di pagare?"
I hope this clears up any confusion, and I apologize for any inconvenience caused. If you have any more questions or need further clarification, feel free to ask.
Grazie e buona giornata! (Thank you and have a good day!)