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.
John            hesitated.  
Subject        Verb
Agent          Action 

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.
Subject                      Verb
Patient                      Action     

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