In the active voice the subject performs the action in a sentence. The subject is placed at the beginning so that it is the focus. "The girl rode the bike," is an example of active voice.
In the passive voice the subject is the recipient of the action. The subject is placed at the end of the sentence or omitted. "The bike was ridden by the girl," is an example of passive voice. Passive voice sentences include a be-verb such as am, is, was, were, are or been. However, the use of a be-verb does not always mean a sentence is in passive voice. "The girl is riding the bike," is an example of a sentence that uses a be-verb but is not passive.
Using active voice makes writing easier to understand. Active voice clearly expresses who or what performed the action and who or what was the recipient. It helps limit the number of words in a sentence so that the writing is clear and concise. Active voice also makes writing seem more forceful and authoritative.
Passive voice is a useful means of conveying indirect expression. Writers use passive voice where the subject performing the action is obvious, unknown or unimportant. Likewise, passive voice allows a writer to emphasize the action rather than the subject.
Scientific writers use passive voice because it allows them to write without the use of pronouns or names as the subjects of sentences. This allows the writer to seem objective and gives the perception that the writing is rooted in fact rather than opinion or personal interest.
However, there are some drawbacks to using passive voice. Passive voice makes sentences seem awkward, wordy and difficult to understand. Passive voice also makes writing seem uninteresting and creates the perception that author is not confident in his writing.
According to the University of Minnesota, writers should use active voice the majority of the time. However, occasional use of the passive voice can add variety to a writing. There are also instances in which passive voice is appropriate. Passive voice is appropriate where the writer wishes to focus the reader's attention on the process of a specific action rather than the person or thing performing the action.