In the active voice, the subject does the action; in the passive voice, the subject receives the action. Although both voices are grammatically correct, the active voice is usually more effective because it is clearer and more direct.
ACTIVE
Hernando caught the fly ball.
PASSIVE
The fly ball was caught by Hernando.
In passive sentences, the actor (in this case, Hernando) frequently does not appear: The fly ball was caught.
Most of the time, you will want to emphasize the actor, so you should use the active voice. To replace a passive verb with an active one, make the actor the subject of the sentence.
The decision to use the active or the passive voice will be influenced by your purpose but also by your audience’s expectations. In much scientific writing, for example, the passive voice properly emphasizes an experiment or a process, not a person: Just before harvest, the tobacco plants are sprayed with a chemical to prevent the growth of suckers.