/ Nobody swept this street yesterday.
->This street wasn't swept yesterday
2/ Somebody has already told her the whole truth.
->She has been told the whole truth
3/ The police found two children in the forest, didn't they?
->Two children were found in the forest, weren't they?
4/ A thief had stolen her car before you arrived.
->Her car had been stolen before you arrived
5/ They used to start these engines by hands.
->These engines used to be started by hands
6/ No one has seen Steve since the day of the party.
->Steve hasn't been seen seen the day of the party
8/ A young man sent those letters for me.
->I had a young man sent those letter for me.
9/ You ought to water the plants every day.
->The plants ought to be watered every day
10/ People had reported the accident before I phoned.
->The accident had been reported before I phoned 
11/ Tom has just invented an armchair which can play music.
-> An armchair which has just been invented can play music
12/ We are going to take our children to the zoo next Sunday.
-> Our children are going to be taken by us to the zoo next Sunday
13/ Dangerous driving cause many accidents.
-> Many accidents are caused by dangerous driving
14/They didn't hold a meeting in the village hall last week.
->A meeting wasn't held in the village hall last week
15/ The scientists are studying the problem very carefully.
->The problem is being studied by the scientists very carefully