Definitions for "active" and "passive" verbs are similar to their names. A verb is an action or a state-of-being word. When a verb is "passive," its subject does not perform the action. A verb is "active" when the subject performs the action.
Identify the components of the sentence. For instance, "The bedroom light was turned off by Jim." The complete verb phrase is "was turned." The subject is "light." Now ask if the subject (light) is completing the verb (was turned).
Can a light turn itself off? It cannot, so rearrange the sentence so the subject completes the verb: "Jim turned off the bedroom light."
The preposition "by" follows most passive verbs. To change the verb from passive to active, the object of the preposition normally becomes the sentence's subject. In the above example, the object of "by" is "Jim," which is the correct subject of the sentence with an active verb.