Question

Devoir Écrit

Answer 1

La voix active is when the subject does the action, instead of receiving the action. In this case all you have to do switch the object and the subject, and change the verb tense from passé surcomposé to passé composé.
The first one becomes : Ils ont réparé ma voiture.
The second one : Mon fils a perdu son chat.
You see that there's one past participate that is remove and the sentence seems to be flip (kinda). Also, sometime you have to make up the subject since it's not explicit like in number 1.
3 Le technicien a réparé notre ordinateur.
4. Ici, nous avons vendu du café. 
5. Nous avons ouvert la porte ce matin.
6. Nous avons fermé les livres.
7 Le prof a corrigé ma composition.
8. Ils ont bâti la maison en 1955.

Answer 2

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 :)