Passive voice

In the active voice, the subject of the sentence DOES the action:

John reads a book. 

Subject / verb / object

In the passive voice, the subject of the sentence is acted upon:  A book is read (by John). 

Subject / verb

Notice that the object of the active sentence (book) became the subject of the passive sentence.  Note: we use the past participle form of a verb in constructing a passive voice sentence and question. 

PASSIVE VERB TENSES

PRESENT SIMPLE

ACTIVE VOICE: She eats an apple.

PASSIVE VOICE: The apple is eaten.

PAST SIMPLE

ACTIVE VOICE: She ate an apple.

PASSIVE VOICE: The apple was eaten. 

FUTURE SIMPLE

ACTIVE VOICE: She will eat an apple.

PASSIVE VOICE: The apple will be eaten. 

PRESENT CONTINUOUS

ACTIVE VOICE: She is eating an apple.

PASSIVE VOICE: The apple is being eaten. 

PAST CONTINUOUS

ACTIVE VOICE: She was eating an apple.

PASSIVE VOICE: The apple was being eaten.

PRESENT PERFECT

ACTIVE VOICE: She has eaten an apple.

PASSIVE VOICE: The apple has been eaten.

PAST PERFECT

ACTIVE VOICE: She had eaten an apple.

PASSIVE VOICE: The apple had been eaten.

MODALS

ACTIVE VOICE: She must eat an apple.

PASSIVE VOICE: The apple must be eaten. 

USE: 

The Passive is used: 

When the agent (= the doer of the action) is unknown, unimportant, or obvious from the context. 

For Example,

I study Economics at the university. 

She works in an English academy. 

They eat vegetables every day.