An ELT Glossary : Voice - Active and Passive
Richards (1985:308) defines voice as : The way in which a language expresses the relationship between a verb and the noun phrases which are associated with it. In English there are two types of voice : active and passive.
Active voice indicates that the noun phrase which acts as subject of the action is also the agent ("doer") of that action, while the patient (recipient) of the action appears as the object of the verb, For example :
The new company demolished most of the old buildings
Subject Verb Object
When passive voice is used, on the other hand, it is the patient of the action which acts as subject.
Most of the old buildings were demolished.
As this example shows, passive voice is formed using the relevant form of the auxiliary verb BE plus the past participle of the main verb
The use of passive voice allows the speaker/writer to omit reference to the agent of the action, as in the example above. This may be eg because the agent is unknown :
Our bank was robbed last night.
or because it is is obvious from the context :
We had really strong winds last week and a lot of trees were blown down.
Here the previous reference to the winds, plus the meaning of blow makes it obvious that it was the winds that blew the trees down - it would be redundant to restate it.
However, the passive may also be used when the speaker/writer does want the agent to be mentioned, but wants the patient of the action to receive the main focus in the clause. In this case the agent can be mentioned in a prepositional phrase using by :
Most of the old buildings were demolished by the new company.