Passive Voice

When we change a sentence from the active into the passive......

1) The object of the active sentence becomes the subject of the passive.

2) We use the verb TO BE in the same tense as the active verb and the past participle of the main verb.

3) The subject of the active sentence becomes the agent and it is preceded by BY.
(The agent is omitted when the subject of the active sentence is one of the following words: somebody, someone, people, he, she, they, etc)

Active sentence: A lot of children use computers nowadays.

Passive sentence: Computers are used by a lot of children nowadays.

Some verbs take two objects like give, send, sell, offer, pay; so we could have two passive forms.

Active Sentence:
Someone gave Mary a letter.

Passive Sentence:
Mary was given a letter. (This sentence is more usual)
A letter was given to Mary.

When do we use passive voice?

1) When the doer of the action is unknown, obvious or unimportant.

My watch was stolen.
The thieves were arrested.

2) When the action is more important than the person who carries it out.

A man was killed in a car crash yesterday.

Let’s see a table of active tenses and their passive equivalents.
Active --------------------- Passive

He takes photos.------ Photos are taken.
He is taking photos.--- Photos are being taken.
He took photos. -------Photos were taken.
He was taking photos. -Photos were being taken.
He has taken photos. --Photos have been taken.
He had taken photos. --Photos had been taken.
He will take photos. ----Photos will be taken.
He would take photos. -Photos would be taken.
He has to take photos. -Photos have to be taken.
He must take photos. --Photos must be taken.
He can take photos. ----Photos can be taken.
He could take photos. --Photos could be taken.
He may take photos. ---Photos may be taken.
He might take photos. --Photos might be taken.

Exercise 1
Exercise 2
Exercise 3
Exercise 4
Exercise 5
Exercise 6