When should a passive voice be used?

The passive is formed with this pattern:

be + past participle

for example: is used / was wanted / is being built

Passive is the opposite of active. In most sentences/clauses the subject refers to the doer or actor of the action of the verb.

Eg. The cat chased the mouse. The cat is the subject or the doer of the verb, chase (active).

But the passive form allows us to put someone or something that is not the actor first in the position of the subject.

Eg. The mouse was chased by the cat.

If you want to say who does the action of a passive verb add - by + noun phrase - after the verb phrase. The noun phrase following by is called the agent.

The passive with agent allows us to say the actor at the end of the clause. This is useful:

1. If the actor is the most important piece of information:

The painting is very valuable. It was painted by Van Gogh.

2. If the actor is described by a long phrase which could not easily be the subject:

The school will always be remembered and supported by the boys and girls who received their education here.

(here the agent is a long noun phrase [bold letters] and would be awkward as the subject).

The passive allows us to leave the actor out, if the actor is not important.

active - Leonardo da vinci painted The Mona Lisa in the 16th century.

passive - The Mona Lisa was painted in the 16th century.

Passive with out the agent is often used when desrcibing a process where it does not matter who or what does the action eg

Tea is picked and taken to the Factory. It is dried and packed into boxes