An ELT Glossary : Agent / Patient
Agent and patient are two terms which describe how the subject and object of a verb relate to an action described by the verb. In simplistic terms, the agent is the person/thing which performs the action, while the patient is the person/thing affected by the action.
This means that in an active sentence using a finite transitive verb, agent and patient are expressed by the subject and object of the verb respectively :
The police arrested the man.
Someone has cut down the old oak tree
Subject Verb Object
Agent Action Patient
Obviously, if the verb is intransitive, the sentence will have no patient :
The plane took off.
And notice that not every finite active verb has an agent - although they will always have a subject. Consider expressions like It rained / It snowed. Here a subject is obligatory grammatically, but it makes no sense to ask What rained? What snowed? The verb describes a process that has no specific agent. Thus the "dummy" subject it is used.
English also allows the agent to be omitted by the use of the passive - in a passive construction, the patient becomes the subject.
The man was arrested.
The old oak tree has been cut down.
This places the emphasis on the patient, and is useful when the agent is obvious (as in the first example), unknown (as possibly in the second) or simply irrelevant (again as possibly in the second example. However, if we do want to include the agent, this can still be done by including it in a prepositional phrase with by, which acts as an adverbial in the clause:
The man was arrested by the police.
Subject Verb Adverbial
Patient Action Agent