When to Use the Passive Voice

It is good academic and business/professional style to write in the active voice. The active voice helps the writer clearly identify who or what is the agent (the subject-actor) of the sentence. The active voice also encourages the writer to use more specific verbs. However, it is never a good idea to write all of your sentences in a single style because doing so may produce a rhythmic effect that dulls the attention of the reader. One of the ways of varying your sentence style is by using the passive voice.
However, when the writer doesn't know who the agent is or if the agent has already been clearly established or is generally agreed upon to both the writer and the readers, the passive voice should be used.
Examples:

1. A newspaper article covering a fire in a warehouse may say:
   "The Fire Chief suspects that the fire was started by an arsonist."
   In this case, the exact arsonist is not known.
   
   
2. An academic article on the Christian Crusades may say:
   Towns were routinely burned to the ground and residents driven from their homes.
   In this case, the reader is well aware that the agents are Christian Crusaders.

But there are also rhetorical reasons for using the passive voice. In these cases, the active voice could have been chosen, but the writer chooses the passive voice to take attention away from the agent.

1. A writer may use the passive voice to intentionally obscure (hide) the responsible agent. In business or professional writing, a company representative or a professional may have to deliver bad news to a client, an employee, a potential employee, a colleague, or a customer and may want to avoid taking responsibility or blame for an undesirable action. A business letter delivering bad news may begin with a sentence such as:
   "I regret to inform you that your application for employment is no longer being considered."
   In this case, the writer wants to obscure who is responsible for making the decision so that the recipient does not know who to blame.
   
2. A writer may also use the passive voice when he or she wants to emphasize what is being spoken about, not the agent. Example: "The event was built around six sessions, with shifting panels of participants doing brief presentations on the subject of the session.
   In this case, the writer wants to focus attention on the event, not who "built" it because he want to the reader to focus on how the event was organized.
   
3. A writer may be attributing a feeling, attitude, or belief to an entire group of people and uses the passive voice to avoid naming the members of this group. Example: "Weapons of mass destruction were believed to exist in Iraq."
   In this case, the writer wishes to express that a belief existed in general. The writer may use a sentence like this to introduce the evidence that the statement is valid. But writers also often use the passive voice to avoid having to demonstrate the truth of the statement because they consider that its truth is general knowledge. While such use of the passive voice is common, it can present ethical problems. Such statements may leave the readers feeling that because "everybody" believes something, it must be true.