With active voice, the subject performs the action expressed in the verb; a direct action. Sentences with active verbs are generally clearer, more direct, and more concise.
With passive voice, the subject is acted upon. Who performs the action may appear in a “by the...” phrase or may be omitted. Passive voice always includes a form of to be, such as am, is, was, were, are, been, or of course, to be in the verb construction. Overuse of passive verbs is often overly wordy, flat, and slows the pacing.
Active voice: The dog bit the girl.
Passive: The girl was bitten by the dog.
Active: Alan will submit his manuscript to the publisher.
Passive: The manuscript will be submitted to the publisher by Alan.
Active: Scientists have conducted experiments to test the theory.
Passive: Experiments were conducted by scientists to test the theory.
Rules of thumb
To avoid overuse of passive voice, I do a search on the word ‘was’ if writing in past tense and ‘is’ when writing in present tense. I try to limit the number of times I use these passive voice words to three per page.
What is really confusing is when a writer starts a sentence I in active voice, then changes to passive, as in: Many regular customers found the coffee too weak to enjoy, but it was still ordered frequently. Edited: Many customers found the coffee too weak to enjoy, but they still ordered it frequently.
Changing passive voice to active voice
To change voice from passive to active, consider who or what is performing the action. Make that who or what the subject of the sentence and change the verb accordingly.
Passive: The movie is being reviewed by every reviewer.
Active voice: Every reviewer is reviewing the movie.
Using passive voice effectively
The passive voice is effective when the subject performing the action is obvious, unimportant, or unknown or when a writer wishes to postpone mentioning the subject until the last part of the sentence or to avoid mentioning the subject at all, thus highlighting the action rather than who performs it.
Active: “Authorities make rules to be broken,” he said defiantly.
Passive: “Rules are made to be broken,” he said defiantly.