When to use Passive Voice
To understand the correct use of passive voice, you must first understand what it is. Read about transitive and intransitive verbs and then action and static verbs. Then, read this article about how and why students use passive voice.
Although you should emphasize action verbs, use passive voice in three situations.
A. Use passive voice when the paragraph focuses on the recipient of the action (the kickee) or when needed for cohesion (i.e., when needed to maintain a consistent topic string). This is a difficult but important point. Consider the following examples.
Many species of nematodes live in soil. Some species are consumed by nematode-trapping fungi. This paragraph focuses on nematodes rather than fungi. Therefore, you probably would use passive voice in the second sentence (as above) rather than active voice (as below).
Many species of nematodes live in soil. Nematode-trapping fungi consume some species. The active voice is more vigorous in sentence two but it changes the focus and reduces the coherence.
Maintaining coherence may require frequent shifting between active and passive voice. See 'Style: Ten lessons in clarity and grace' for more examples.
B. Use passive voice to replace a long subject with a short one.
The large numbers of nematodes in the soil and the history of crop failure supported verb our decision to fumigate (active, but verb appears too late).
Our decision to fumigate was supported verb by the large numbers of nematodes in the soil and the history of crop failure (passive, but verb appears reasonably soon).
C. Use passive voice when the actor (the kicker) is irrelevant or unknown.
The kicker is irrelevant in this sentence: The fumigant must be stored in a locked and posted building.
Scientists often use passive voice when describing methods: The sample was spun at 1000 g in a centrifuge. After the pH was adjusted, the supernatant was placed on an agarose gel and subjected to an electric current. vs I spun the sample at 1000 g in a centrifuge. I adjusted the pH of the supernatant and placed it on an agarose gel. I then ran an electric current through the gel.
Here are three letters about using active and passive voice; the letters were published in the journal Nature.