In sentences using active verbs, a noun performs the action of a verb, while in passive voice sentences, the verb is acted upon by the noun. For example, a sentence containing an active verb is "the professor teaches the student," while a passive verb sentence is "the student is taught by the professor."
In active voice, the person or thing performing the action of the verb is placed at the front of the sentence. This noun is the subject of the sentence. If the verb action is directed at another person or thing, that object is placed at the end of the sentence. This is the target of the active verb. For example, in the sentence "John keeps butter in the fridge," "John" is the subject of the sentence, "butter" is the target and "keeps" is an active verb.
In the sentence "the butter is kept in the fridge," the original verb target, "butter," is moved to the beginning. The passive verb "is kept" acts upon the noun "butter." Passive voice may be used if the speaker wishes to emphasize the object of the sentence rather than the subject. In general, active voice is more straightforward and professional than passive voice.