Active voice is a term used to identify a style of verb usage in the English language. When a verb is considered active, the subject of the verb performs some action on the object. The alternative is considered the passive voice, where the subject and object are reversed, and the subject receives the action. In this way, active verbs are often considered to be more vibrant and descriptive than their passive alternatives and give the subject of a sentence the proper focus.
Transitive verbs — verbs whose subjects require an object in order to form a complete sentence — are the only type of verb capable of having voice. In the example sentence, "Jane slapped Tommy," the verb "slapped" is used in the active voice to show what action Jane took upon Tommy. Alternatively, in the passive voice, "Tommy was slapped by Jane," comes out longer and changes the focus of the sentence from Jane to Tommy.
Given the simple examples above, some notable aspects of the active voice emerge. In most cases, passive verbs are some form of the verb "to be," such as "is, was, were, been." If a reader looks for one of these inflections, it can quickly reveal whether the sentence is using the active or passive voice. Sentences, or sentence clauses, that are written in the active voice also remove a preposition, which reduces overly wordy sentences from a particular composition. Converting a passive sentence to one in the active voice, then, is often a matter of switching the subject and the object, removing the "be" verb and its associated preposition, and giving the subject a verb that describes what it is doing to the object.
There are cases, however, when the verb "to be" will be found in sentences using the active voice. Consider the example sentence, "Tommy pulled Jane's hair." In what's called a progressive conjugation, the verb "to be" can be used with the present participle of a verb that still shows the subject performing the action. Using progressive conjugation, the above example would then become, "Tommy is pulling Jane's hair." Both sentences are showing the action that Tommy, the subject, is performing upon Jane, even though the second sentence uses an inflection of "to be."
Progressive conjugation is not the only way the passive and active voice might be confused. Sometimes, the passive verb is implied in a sentence. Take, for example, the sentence, "The slap given by Jane taught Tommy a valuable lesson." There is an implied "that was," which if written before the verb "given," would make the sentence overly wordy. Even though the inflection of "to be" doesn't actually appear in the sentence, its implication still gives the sentence clause a passive voice.