Although this is often correct. There are many sentences in which the noun identified as subject is not strictly a instigator or agent performing the action indicated by the verb. An agent is something that itself can take action. In sentences of the class: a) The knife cut the rope b) The stone broke the window c) The brush painted flowers d) scared the dog statue e) The old man scares him f) Rosa became a skirt g) The dictator killed prisoners Regular practice is to identify the nouns (cuchillo. Stone, Brush, statue, elder, Rose, dictator as grammatical subjects of their prayers. But it is fair to say that all are agents that perform the actions mentioned by the verb of each sentence. The knife elements, stone, brush, statue, they are usually tools used to perform actions or processes, by itself does not perform any action. In the example e) the elderly is the subject-agent if you have something that caused fear in children.
If he did not act voluntarily instilled fear of children, then the old man is an agent, but it worked as a subject-object causing the emotional change in children. F In prayer, Rosa may be the subject-agent if she did the work of measuring, cutting and sewing fabric. But if she paid for a seamstress or tailor to do the job, then it is subject-benefactive (possessor or winner) in the process. In the latest example of the dictator may be the subject-agent, if he killed the prisoners, but it is possible that he personally has not killed the prisoners.