If there’s one thing people remember from their high school English classes, it's that you should always strive to use the active, rather than the passive, voice in your writing. But like most of the "rules" of writing (and a few of the rules of grammar) this isn't really a rule. Some times the passive voice is appropriate, and some times it is even needed.
So what are the passive voice and active voice?
A basic sentence has a subject, an object and an action.
In a passive sentence, the object receiving the action is the subject of the sentence and the thing doing the action is included near the end of the sentence. You use the passive form when you thing the thing receiving the action is more important and should be emphasized. You would also use the passive form if you don’t know who is doing the action, or if you don’t want to mention who’s doing the action.
In the most basic active-voice construction is: subject – verb – direct object. The subject “acts” on a direct object. A verb is in the active voice when it expresses an action the subject performs.
Verbs in the passive voice have two parts: some form of the verb “to be” and a past participle form of the action verb: was thrown. (The helping verb has or have can also appear in a passive verb: the ball has been thrown.)
Active voice is usually clear, emphatic, and flowing. It is also direct. Readers prefer the active voice (whether they are aware of it or not), because it decreases the amount of mental work required for understanding the text. People naturally have a more positive reaction to active voice than they do to passive voice.
William Zinsser (who wrote On Writing Well) says that passive voice should be used sparingly–only when there’s no way around it. “The difference between an active-verb style and a passive-verb style–in clarity and vigor–is the difference between life and death for a writer.”
Active voice is the voice of power, action, and drive. It is the voice God speaks in. God didn't say, "Thy neighbour’s wife shall not be coveted by you." He said, "Thou shalt not covet thy neighbour's wife." If God wrote the Ten Commandments in the passive voice, they would sound more like the Ten Suggestions. When He writes in an active voice, you know He means business.
In most (but not all) cases, the passive construction is longer, clunkier, and more vague.
Writing in the active voice rather than the passive voice gives your writing more life and more clarity. Use the active voice unless doing so makes a sentence awkward. Your writing will stand out, and your readers will perceive you as more energetic than if you had used passive voice. No matter what field you are in, active voice will improve your credibility because you are talking to the reader instead of at the reader.
There are, however, sometimes good reasons to use the passive voice. A writer may choose to use the passive voice in order to emphasize one thing over another or to emphasize the action rather than the actor. It can be used to keep the subject and focus consistent throughout a passage. Use a passive voice if you’re trying to be tactful by not naming the person performing the action or to describe a condition in which the actor is unknown or unimportant.
The use of active voice over passive voice is, in part, a matter of word economy and simplicity. If you can say something with fewer words, you probably should. It’s also a matter of making your words work for you. As Zinsser says, “active verbs push hard and passive verbs tug fitfully.” Using an active verb helps make the sentence more vivid and precise; does your subject walk, or does he saunter? Does she fall, or does she stumble?
Julius Caesar loved Cleopatra. This is an active sentence.
Julius Caesar is the subject, the action is loved, and the object is Cleopatra.
Cleopatra was loved by Julius Caesar. This is a passive sentence.
Cleopatra is the subject, was loved by is the passive action.
Although Cleopatra is the subject of the sentence, she isn’t really doing anything. Instead, she’s the recipient of Caesar’s love. The focus of the sentence has changed from Caesar to Cleopatra.
Cleopatra was loved. This is also a passive sentence.
Cleopatra is still the subject, but she’s been given more importance because we’ve taken Caesar out of the picture.
As you can see by the above examples, sometimes a passive voice can be just as effective as an active voice. It’s up to you, as the writer, to decide which one is called for. While you don’t want to overdo the use of the passive voice, neither do you want to leave it out completely. Strike a balance between the two and your writing will be more powerful.