A verb's voice indicates the subject's relation to the action. In English, this is whether the subject is acting, or being acted upon. There are two voices, an active voice and a passive voice. This can be thought of as looking at the same event from opposite directions.
For example, take the case of a dog biting a man. We can say this in the active form: "The dog bit the man." Alternatively, we can turn it to a passive form, and say "The man was bitten by the dog." These sentences are the same in meaning. They have a slight difference in emphasis.
- (Note that "The dog was bitten by the man" is not equivalent. It is the same as "The man bit the dog" and has the opposite meaning to the earlier examples - and a much less common one.)
Voice uses an auxiliary verb to modify the verb (similarly to aspect), most usually with the verb 'to be': this is the 'be-passive'. In informal English, it is often replaced by 'get' - the 'get-passive'. Do not use the get-passive in Academic English writing. (Indeed, avoid the word 'get' as much as possible.)
Combined with different aspects and tenses, this gives examples like 'the window was broken by the ball' (simple past passive), 'that car is being driven by a madman' (present continuous passive) and 'my assignment will be finished before Saturday' (future simple passive). In all these examples, the logical Object - the person or thing on whom the action of the verb is performed - becomes the grammatical Subject. The logical Subject does not have to be expressed at all; indeed, one use of the passive can be to conceal responsibility. "What happened to the paint?" says the teacher. "It was spilt," says the pupil - who will not say by whom it was spilled (an agentive phrase).
- Teachers in science subjects have often encouraged or insisted upon their students using the passive voice, particularly when writing up experiments. Students are told to say 'The burner was lit' rather than 'I lit the burner'. This seems to AWE to be an unnecessary step towards objectivity, or, more accurately, towards pseudo-objectivity. See Objectivity in Science writing.
Note that only transitive verbs have a passive. Intransitive verbs, by definition, do not have an Object - a logical Object. So they have nothing that can be transformed into the Grammatical Subject of a passive verb. (Verbs that can have both a transitive and an intransitive form can have a Passive - but only for senses in which they are used transitively. That should be obvious - but it may take some concentrated thought from readers who are not used to grammar.)