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In linguistics, a grammatical agent is the participant of a situation that carries out the action in this situation. Also, agent is the name of the thematic role (also known as the thematic relation) with the above definition. The word comes from a participle of the Latin verb, agere.
For example, in the sentence "Jack kicked the ball", Jack is the agent. In certain languages, the agent is declined or otherwise marked to indicate its grammatical role. In Japanese, for instance, the agent is typically affixed with |ga| (the hiragana が). Although Modern English does not mark grammatical role, agency is informally represented using certain conventions; for instance, with the morphemes "-ing", "-er", or "-or", as in "eating", "user", or "prosecutor". (Cf. agent noun.)
The notion of agency is easy to grasp intuitively but notoriously difficult to define: typical qualities that a grammatical agent often has are that it has volition, is sentient or perceives, causes a change of state, or moves. These are in fact the qualities that Dowty included in his definition of a Proto-Agent, and according to his theory, the nominal with the most elements of the Proto-Agent and the fewest elements of the Proto-Patient tended to be treated as the agent in a sentence. This solves problems that most semanticists have with deciding on the number and quality of thematic roles. For example, in the sentence His energy surprised everyone, His energy is the agent, even though it does not have most of the typical agent-like qualities such as perception, movement, or volition.
The grammatical agent is often confused with the subject, but these two notions are quite distinct: the former is based explicitly on its relationship to the verb, whereas the latter is based on the flow of information, word order, and importance to the sentence. In a sentence such as "The boy kicked the ball", "the boy" is the agent and the subject. However, when the sentence is rendered in the passive voice, "The ball was kicked by the boy", "the ball" is the grammatical subject, but "the boy" is still the agent. Many sentences in English and other Indo-European languages have the agent as subject.