Sam built the house (active) The house was built by sam (passive)
In the active voice of the sentence, "Sam" is the subject, clear and simple. In the passive construction, what is then the function of the prepositional phrase "by Sam"? I read that "by sam" in the passive construct functions as an adverbial. My understanding is that adverbs/abverbials provide information about the verb, specifically time (when), place (where), manner (how), degree (to what extent), condition, reason (why/purpose). If "by sam" is an adverbial, then what is it telling me about the verb (i.e. which adverb category (when, why, how, etc) does it fall under? It seems to be stating "who" built the house, but "who" is not a category within the definition of an adverb. If "by Sam" in this sentence is not an adverbial, what is its function in the sentence?