We use passive active voice when the object is more important than who does the action.

Examples:

Several people were injured in the accident.

I was bitten by a dog when I was a child.

We use passive voice when we don't know know who does the action.

Examples:

My keys were stolen yesterday.

This church was built in the 19th century.

We use passive voice when we can can't understand who does the action from the context.

Examples:

The criminals will be arrested soon. (by the police)

How often is your car repaired? (by the mechanic)

We use by to add the major agent to a passive voice sentence.

Examples:

The telephone was invented by Alexander Bell.

The letters have been written by my friend.

We don't include the doer (agent) when:

1. The person who performs the action is isn't important.

   Example: Three million cars are exported every year.

2. The doer (agent) is isn't obvious or already known.

   Example: English is spoken in the UK.

3. The doer (agent) is isn't known.

   Example: The car was scratched when it was in the car park.