Using passives

The choice between an active and passive sentence allows us to present the same information in two different orders.
Compare:

active 

• The storm damaged the roof.
This sentence is about the storm, and says what it did. [The storm is the 'agent'.)

passive 

• The roof was damaged by the storm.
This sentence is about the roof, and says what happened to it. (The 'agent' goes in a prepositional phrase with by after the verb.)
Here are some situations where we typically choose a passive rather than an active.
• When the agent is not known, is 'people in general', is unimportant, or is obvious, we prefer passives. In an active sentence we need to include the agent as subject; using a passive allows us to omit the agent by leaving out the prepositional phrase with by:
• My office was broken into when I was on holiday, (unknown agent)
• An order form can be found on page 2. (agent = people in general)
• These boxes should be handled with care, (unimportant agent)
• She is being treated in hospital, (obvious agent; presumably 'doctors')
• In factual writing, particularly in describing procedures or processes, we often wish to omit the agent, and use passives:
• Nuclear waste will still be radioactive even after 20,000 years, so it must be disposed of very carefully. It can be stored as a liquid in stainless-steel containers which are encased in concrete. The most dangerous nuclear waste can be turned into glass. It is planned to store this glass in deep underground mines.
• In spoken English we often use a subject such as people, somebody, they, we, or you even when we do not know who the agent is. In formal English, particularly writing, we often prefer to use a passive. Compare:
• They're installing the new computer system next month.
• The new computer system is being installed next month, (more formal)
Notice also that some verbs have corresponding nouns. These nouns can be used as the subject of passive sentences, with a new passive verb introduced:
• The installation of the new computer system will be completed by next month.
• In English we usually prefer to put old information at the beginning of a sentence (or clause) and new information at the end. Choosing the passive often allows us to do this.
Compare these two texts and notice where the old information (in italics) and new information (in bold) is placed in the second sentence of each. The second text uses a passive:
• The three machines tested for the report contained different types of safety valve. The Boron Group in Germany manufactured the machines.
• The three machines tested for the report contained different types of safety valve. The machines were manufactured by the Boron Group in Germany.
• It is often more natural to put agents (subjects) which consist of long expressions at the end of a sentence. Using the passive allows us to do this. So, for example:
• I was surprised by Don's decision to give up his job and move to Sydney. is more natural than 'Don's decision to give up his job and move to Sydney surprised me.'