Hi
As in English with the passive voice, you have to put back i the active voice, with a subject before the verb which will be active :
you also have to pay attention to the tenses used.
A past composed tense = passé composé
a passé composé tense = present tense (as in English)
(sequence of tenses = concordance des temps)
1- On a réparé ma voiture
2- Mon fils a perdu mon chat
3- Le technicien a réparé l'ordinateur
4- On vend du café ici.
5- On a ouvert cette porte ce matin
6- On a fermé les livres
7- Le prof a corrigé ma composition
8- On a bâti la maison en 1995.
When the subject is unknown, the best is to use "on" , third person singular.
ON = quelqu'un (someone), / we, they........????? No one knows....It's abstract
could be singular, plural (but conjugated third person singular)
hope I helped :)