The sentence "the tree was planted by Mary" uses the passive voice. When said in the active voice, the sentence will read "Mary planted the tree." A sentence written in the passive voice is where the noun of the sentence is the object instead of the subject. In our case, "Mary" is the noun. In the active voice, "Mary" is the subject and in the passive voice "Mary" is the object. The subject of the sentence is normally the recipient of the action. In our example, the recipient of the action is "the tree".
