a man from Boston - This is a noun phrase. In this grammatical context the noun phrase, "a man from Boston", is the "agent". The man from Boston performed the action of giving someone the lovely old sailor's jacket.
by - The preposition "by" answers the question "how". The man from Boston caused someone to receive or obtain the sailor's jacket. That's how someone was able to obtain or receive the sailor's jacket.
by - preposition
a man from Boston - noun phrase, agent in a passive voice structure, object of the preposition "by"
by a man from Boston - prepositional phrase
agent: West's Encyclopedia of American Law (Full Article) from Answers.com
Linguistics. The noun or noun phrase that specifies the person through whom or the means by which an action is effected.
What is agent as a semantic role?
In these example sentences, the agents are the boy and the dog.
- The boy ran down the street.
- He was chased by the dog.
Student or Learner