In sentences written in passive voice, the subject receives the action expressed in the verb. The agent performing the action may appear in a "by…" phrase or may be omitted.
For example: The boy was bitten by the dog.
(Correction: The dog bit the boy.)
Research will be presented by Dr. Smith at the conference.
(Correction: Dr. Smith will present research at the conference.)
Experiments have been conducted to test the hypothesis.
(Correction: Scientists have conducted experiments to test the hypothesis.)
You can recognize passive-voice expressions because the verb phrase will always include a form of be, such as am, is, was, were, are, or been. Another way to recognize passive-voice sentences is that they may include a "by…" phrase after the verb that names the agent performing the action.
In scientific writing, passive voice is more readily accepted since using it allows one to write without using personal pronouns or the names of particular researchers as the subjects of sentences (see the third example above).
In most nonscientific writing situations, however, active voice is preferable to passive for the majority of your sentences for several reasons. Often, the use of passive voice can create awkward sentences. Also, overuse of passive voice throughout an essay can cause your prose to seem flat and uninteresting.
Even in scientific writing, overuse of passive voice or use of passive voice in long and complicated sentences can cause readers to lose interest or to become confused. Sentences in active voice are generally – though not always – clearer and more direct than those in passive voice.
For example: The difficult entrance exam was failed by over one-third of the applicants to the school. (passive/indirect)
Over one-third of the applicants to the school failed the difficult entrance exam. (active/direct)
Sentences in active voice are also more concise than those in passive voice because fewer words are required to express action in active voice than in passive.
For example: Action on the bill is being considered by the committee. (passive)
The committee is considering action on the bill. (active)
Changing passive to active
If you want to change a passive-voice sentence to active voice, find the agent in a "by…" phrase, or consider carefully who or what is performing the action expressed in the verb. Make that agent the subject of the sentence, and change the verb accordingly.
For example: The book is being read by the class. (passive)
Compiled and edited from Purdue Online Writing Lab (OWL), http://owl.english.purdue.edu