When the action is more important than the agent who performs the action.
To introduce a scientific fact.
When it is obvious who the agent is.
When people in general are the agents.
When we don't know the agent who performs the action.
Remember the trash is collected on Fridays.
My wallet was stolen yesterday.
Only the books with green stickers can be borrowed from the library.
My neighbor was arrested last night.
The ozone layer is being destroyed by CFCs.
