The term ‘active voice’ describes how your writing focuses on the items you discuss. Use active voice to define agency (who or what does the action). If you use transitive verbs and put focus on the proactive subject, you keep your writing in the active voice.
Generally, most writing is clearer and smoother in the active voice rather than the passive voice. As a rule of thumb, you should use active voice whenever possible. This keeps your writing more direct by focusing on the proactive subject of a sentence rather than the passive object. A common mistake is switching into the passive voice for dramatic effect. This does not make writing more intelligent or artistic; it just tends to dilute the meaning.
Incorrect: A vodka martini was drunk by the pink elephant with a long trunk and droopy ears. (This sentence focuses on the passive object [the martini] rather than the active subject [the elephant].)
Correct: The pink elephant with a long trunk and the droopy ears drank a vodka martini. (This sentence focuses on the active subject [the elephant] rather than the passive object [the martini].)
There are some situations that may call for the passive voice, but they are few and far between. In some academic contexts, a teacher may instruct you to avoid first person references to yourself. This can lead you to write in the passive voice. If you want to acknowledge a mistake without fully accepting responsibility, the passive voice is an excellent choice. If you ever see a politician giving a press conference after a scandal, pay close attention to how he or she accepts responsibility. Odds are that he or she uses the passive voice as a minimization technique.
Incorrect: The drunk elephant acknowledged to the revelers that some mistakes had been made by him.
Correct: The drunk elephant acknowledged to the revelers that he had made some mistakes.