The function of the prepositional phrase at the tail end of a passive voice sentence is to identity the doer of the action of the operative verb in that sentence, as in the sentence you presented, “The rice is harvested by the farmers.” Such prepositional phrases, as we know, are an optional element in the passive-voice sentence. Passive-voice sentences can stand without them: “The rice is harvested.”
As to your second question: Does the subject of a sentence in the active voice become the object when the sentence is rendered in the passive voice?
Before I answer that question, let’s first clarify what the active voice and the passive voice are in the first place.
By definition, a sentence is in the active voice when its grammatical subject performs the action of the operative verb in that sentence, as in this form of the sentence you presented: “The farmers harvest the rice.” Here, the noun “farmers” is the grammatical subject, “harvested” is the operative verb in the active-voice form, and the noun “rice” is the direct object of that verb.
On the other hand, a sentence is in the passive voice when its grammatical subject receives the action of the operative verb in that sentence, as in the original form of the sentence you presented: “The rice is harvested by the farmers.” Here, the noun “rice” is the grammatical subject, “is harvested” is the operative verb in the passive-voice form, and the noun “farmers” is the doer of the action of this verb.
We can see that the grammatical subject of the active-voice sentence, “farmers,” didn’t become the direct object in the passive-voice sentence. Instead, by becoming part of the attributive prepositional phrase “by the farmers,” the noun “farmers” became what’s known as the object of the preposition. This object of the preposition is optional to the passive-voice sentence. It can be taken out and the sentence can stand without it: “The rice is harvested.”
The answer to your second question is therefore “no.” The subject of a sentence in the active voice doesn’t become the object when the sentence is rendered in the passive voice. It becomes the object of the preposition instead, which is an altogether different thing from the direct object that it was in the active voice. Moreover, as an object of the preposition, it becomes optional to the sentence.
Your second question is whether the phrase “by the farmers” is an adverb phrase that modifies the verb “harvested” rather than an object. The answer is yes, it is functionally an adverb phrase that modifies the verb “harvested,” but grammatically, it’s also the object of the preposition “by” in that sentence.
