The Passive Voice in Past tense changes the meaning of the sentences because we focus on things already done. You have to identify what is thing that was made by someone using the past form of be + Verb in Participle
Active voice: The president opened the building in 1931.
Passive voice: The building was opened by the president in 1931.
Active voice: An American architect designed the building .
Passive voice: The building was designed by an American architect .