When/if we use the passive, it is generally because:
- We don't know who the agent is, or the agent is not important:
Ruth Ellis was hanged in 1955. She was the last woman to be executed in Great Britain.
Here, the person who performed the execution is known to some (but not many) people, but he is unimportant. What is important is the hanging of this woman.
-We may wish not to place importance on the agent:
I'm afraid your application for a loan has been declined.
Possibly the speaker rejected it himself, but would rather not admit this.
- In academic writing, especially in scientific writing, the passive gives an impersonal, objective impression.
The passive is rarely used in informal conversation.
People who use the passive for one of these, or for some other reason, generally produce the sentences in the passive. They do not produce an active sentence and then transform it.
I loathe transformation exercises in coursebooks. They often result in very unnatural sentences.