Page 860 - Wordsmith A Guide to College Writing

In this sentence, Tom, the subject, performs the action described by
the verb ate. Rewritten in passive voice, the sentence looks like this:

The hamburger was eaten by Tom.

What has changed? Simply put, the subject of the verb is not acting,

but is acted upon. The subject hamburger performs no action, but
instead is acted upon by the person who eats it. Notice, too, another

hallmark of the passive voice: The verb contains a helping verb that is

a form of the verb to be. Though an active voice verb also may have a

helping verb such as is, was, were, have been, will be, or another form

of to be, a passive voice verb always has a helping verb. Another

hallmark of the passive voice is the by construction that sometimes
tells who or what acted upon the subject.

Memory Jogger

In active voice, an action is done by the grammatical subject.

In passive voice, an action is done to the grammatical subject.

Practice 21-4 Examining Active

Voice and Passive Voice Verbs

Look at the following sets of sentences and answer the questions
about them. The first one is done for you.