Here you have some exersices to practise the passive voice in present and past simple.
Let's revise first the structure:
Sub + to be (is/are/was/were) + participio + C.Ag (by ....)
The thief was arrested by the police
Buildings aren't made of paper
http://www.clafoti.com/3/pass2.htm
http://www.clafoti.com/3/passive5.htm
http://www.clafoti.com/1.4/24/24.1.htm
http://www.clafoti.com/1.4/25/25.1.htm
