Réponse:

4. the thief was arrested by them

5. the house was decorated by Laura,my sister

6.you will be asked by them to open your cases

7.my bicycle was stoled by someone

8.dogs are not allowed in that shop by them

9.the flat has might be seld by them (pas sûre)

10.if the accident would be seen by the girls....

11.the taxes will certainly soon be reduced by them.