The passive voice is used if you don't know who preformed the action descried in the sentence.
TRUE
They had made many promises.
Many promises had been made by them.
We were invited by our neighbors to attend their party.
What is... Passive.
The police will punish the thief.
The thief will be punished by the police.
They are painting my office today.
My office is being painted today.
Mark hung the decorations.
An American writer wrote this novel.
This novel was written by an American writer.
The ball was chased by the dog.
The dog chased the ball.
"Timothy has kissed Jane." This is written in active voice.
TRUE
Mrs. Mira teaches math.
Math is taught by Mrs. Mira.
The cat had been fed.
Passive
The customer was being helped by the salesman.
What is...The salesman was helping the customer.
"Sam's family was accepted by Justin's family." This is written in active voice.
FALSE
The boy is climbing the cliff.
I bought the baby a doll.
What is... Active.