The active
voice is used when the agent (i.e, the doer of the action) is to be made
prominent. The passive voice is used
when the person or thing acted upon is to be made prominent. The passive voice
is therefore preferred when the doer of the action is an indefinite pronoun or
noun (e.g. somebody, anybody, they, people, we etc.)
My watch has been stolen. (Passive) Somebody has stolen my watch. (Active)
In the example given above the passive structure
is preferred because we don’t know who performed the action.
Compare:
Peter has stolen my
watch. (More natural than ‘My watch has been stolen by Peter.)
Here the emphasis is on Peter and therefore we
use the active form.
In the following cases, the passive forms are
preferred because the subject is vague or indefinite.
I was asked my name. (Passive) They asked me my name. (Active) I was told to pay the fine. (Passive) They told me to pay the fine. (Active) English is spoken all over the world. (Passive) People
all over the world speak English.
(Active)
In
passive clauses, we usually use a phrase beginning with by if we want to mention the agent – the person or thing that does
the action.
I was shocked by his arrogant attitude.
(Passive) His
arrogant attitude shocked me.
(Active) The
spider was killed by John. (Passive) John killed the spider. (Active)
Note that the agent is mentioned in only about
20% of passive clauses.