Hi Laure
6. Ci vediamo davanti AL teatro per le nove. Remember you can also say alle nove. The difference is that “alle nove” literally means “at nine o’clock”, while “per le nove” (around nine o’clock). So they are both good options. You say “al” because gives that feeling of “I’m waiting for you in front of the theatre. I hope it makes sense what I’m saying 😅
“Va pensiero” è stato scritto da Giuseppe Verdi. Da means “by Giuseppe verdi”/
8. Quanti personaggi....? Quanto is for one while Quanti is for quantify more objects.
9. I quadri di Caravaggio mi sono piaciuti molto. — Also in this case, is piaciuti because you loved more paintings.
10. Alla radio si sentono soprattutto canzoni in inglese. — Songs is plural, so you need sentono (third plural person of the verb sentire).
If you have more doubts or questions, please let me know. I’m happy to help :)