A verb not only undergoes certain changes to show tense, or the time of the action, but it changes in form to show voice. Voice is a grammatical term which is used to tell whether the subject of the sentence is acting or is receiving the action by the verb.

When the subject is acting, we say that the subject is the doer. When the subject is receiving the action, we say that the subject is the receiver. If you keep these two terms, doer and receiver, in mind, you will have no difficulty in understanding what voice means in grammar.

Look at the following sentences carefully. Note the changes that occur in the form of the verb, Note the change that occurs in the subject of the sentence:

1. Ned washed the car. Ned is the doer of the action        2. The car was washed by Ned. Car is the receiver of the action

In the first sentence, the subject is Ned. He is the doer, the one who is performing the action expressed by the verb washed. The car is receiving the action. In grammar we say that the verb in this sentence is in the active voice because the subject is the doer, or is doing the washing. The car is the receiver of the action.
The second sentence is written in the reverse order. The subject is now the receiver of the action instead of the doer. In order to express this idea, it was necessary to use another verb form, was washed. What happened to Ned, the doer? Ned is still in the sentence but now in a phrase introduced by the preposition by.

The verb was washed is in the passive voice because it represents the subject of the sentence as the receiver of the action. In other words, the subject is not acting, but is passive. The doer, or the actor, appears in a phrase introduced by the preposition by.

A verb in the passive voice is never a simple verb. It is always a verb phrase. In the sentence, Our car was stolen yesterday, the verb was stolen is in the passive voice. The subject is the receiver of the action. Because the doer is unknown, the "by phrase" is omitted, But we know it was stolen by someone. If we discover who stole the car, the doer might be added to the sentence:
Our car was stolen yesterday by two strangers. (car is the receiver and the strangers are the doers)