In English grammar, the agent is the person or thing that performs an action. In an active (not passive) sentence, the agent is usually the subject.
Jane wrote the letter.
In this example, the subject of the sentence Jane
is also the agent. The object is the letter
. If we change this into a passive sentence, the object is placed at the beginning and becomes the subject.
The letter was written by Jane.
Jane is still the agent but not the subject. The subject is nowThe letter
In a passive sentence, it is not always necessary to include the agent. Imagine another, similar, sentence with more information.
Jane sent the letter yesterday.
We can change this into the passive as follows:
The letter was sent by Jane yesterday.
If it is not important to mention the agent (Jane
) we can leave it out.
The letter was sent yesterday.
This might be quite natural in a real-life situation. Someone might want to know when a letter was sent but not who sent it. This is a common reason for using the passive voice.