Passive Voice

Passive voice is a form of sentence that focus on the action rather than the doer or performer. The subject in passives sentences receives the action (while the subject in active sentences perform the action).

We use passive form when:

• we don’t don't know who/what does the action,
• we don’t want to tell/show who/what does the action,
• who/what does the action is not important,
• or we want to focus only on action.

For examples,

• Active: Navy wrote a letter.
• Passive: A letter was written by Navy.

In the active sentence, Navy (the subject) performed the action (wrote), and a letter (the object) received the action. However, in passive sentence, the a letter (which is now the subject) received the action, and Navy (the agent) becomes object of preposition (by) and is called "agent". See the diagram below: 

NOTE:

[1] The subject in active sentence does the action. It becomes object of preposition or agent in passive sentence.

[2] The subject in passive sentence receive the action. The subject in passive sentence is the object in active sentence.