Once the thing is known, Italian replaces it with lo, la, li or le, placed before the verb. In the passato prossimo the participle then has to agree with that pronoun: la camera becomes l'ho prenotata, i biglietti become li ho comprati.
Examples
- La camera? L'ho prenotata ieri.
- The room? I booked it yesterday.
- La colazione è inclusa, la prendiamo alle sette.
- Breakfast is included, we have it at seven.
- Le chiavi le lascio alla reception.
- I am leaving the keys at reception.
The mistake to avoid
With avere the participle agrees only when one of these pronouns comes first.
Where this fits in
Spostarsi, Italian A2: Muoversi in città e in viaggio: trasporti, indicazioni stradali, albergo e biglietti.