Many sentences describe events that involve two participants: an agent and a theme. And it often happens that the agent role shows up in subject position and the theme role in object position. These sentences illustrate that common pattern: the subjects are all agents and the objects are all themes.
Ilona broke an icicle.
Zainab introduced the guest speaker.
The manager fired the receptionist.
It’s a common tendency across languages for the agent to occupy the subject position, but of course not all agents are subjects, and not all subjects are agents. These next sentences describe pretty much the same events as the last three, but the noun phrases in subject position are not agents.
The icicle broke.
The guest speaker was introduced by Zainab.
The receptionist got fired.
So while the usual pattern is for agents to be mapped onto subject position and themes onto object position, most languages also have a way of reversing that usual mapping. In English, the strategy we have involves both morphology and syntax and is called a passive structure. A passive sentence reverses the usual mapping between thematic roles and grammatical roles.
In this first sentence, The police arrested the burglar, the police are the agent and they’re in subject position, and the burglar is the theme in direct object position.
In the second sentence, The burglar was arrested by the police, the semantic relationship of the police and the burglar to the arresting event is the same: the police are still the agent and the burglar is still the theme. But their grammatical roles are different. We can use this passive structure to reverse the usual pattern and focus our attention more on the theme than on the agent.
The reversal that happens in a passive sentence works the same even if the thematic roles aren’t the classic agent and theme. Take a look at this pair of sentences,
The exhibit impressed the audience.
The audience was impressed by the exhibit.
In the first sentence, which is an active sentence, the usual mapping plays out not with an agent and theme, but with a cause participant in subject position and an experiencer in the object position. When we use a passive structure in the second sentence, the thematic roles of the participants don’t change, but their grammatical roles do.
So how can you tell if a sentence is in the passive voice? It’s easy: a passive sentence will always have some form of the verb be, followed by a past participle. All of these examples are passives.
The burglar was arrested.
The children were invited to the party.
This flight is expected to arrive on time.
The candidate is being prepared for the debate.
I am appalled by your behaviour.
But if you have the verb be plus a present participle, or if you have the verb have plus a past participle, then those aren’t passives. All of these sentences are in the active voice:
The report is calling for changes.
The burglar was planning a heist.
The children were behaving poorly.
The hosts have invited several guests.
The dog had eaten all the Halloween candy.
A passive structure is a morphosyntactic strategy that English uses to reverse the usual mapping of thematic roles onto grammatical roles. Some languages accomplish this reversal with morphology on the verb or with morphology on the noun, but it’s pretty common for a language to have a strategy in the morphology or syntax that has this effect in the semantics of a sentence.