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Passive Overview

 We form the passive using the relevant tense of the verb to be, plus the past participle of the main verb.

So for the verb clean, we would have:

Passive Summary of Tenses
Present Simple
Present Continuous
Past Simple
Past Continuous
Future Simple
Going To Future
Present Perfect
Past Perfect Simple
The room is cleaned every day
It is being cleaned now
It was cleaned yesterday
It was being cleaned at six yesterday
It will be cleaned tomorrow
It is going to be cleaned tomorrow
It has been cleaned twice
It had been cleaned before
It can be cleaned easily
It should be cleaned daily

To form the question, we put the auxiliary verb first:

Is it being cleaned today?
Had it been cleaned before?
Should it be cleaned daily?

How to Use Passive

Look at this sentence:

They will deliver the letter tomorrow.

"the letter" is the object of the sentence. "they" is the subject. We can make "the letter" the subject:

The letter will be delivered tomorrow.

And if we want, we can include the subject of the first sentence:

The letter will be delivered by them tomorrow.

So we use the passive to say what happens to the subject of a sentence.

The bridge was painted in 1999.
Military jets are usually flown by men.
The flight to Boston will be delayed because of the striking ground crew.

And we use an active sentence to say what a subject does:

John Exmoor painted that bridge in 1999.
Air Force pilots, usually men, fly military jets.
Striking ground crew will delay the departure of the Boston flight.

Sometimes, active sentences sound unnatural because who does the action is not important or not known. The action itself is important.

Tickets can be purchased from the booth at the entrance.
Spanish is spoken in much of South America.
Edward Moore was killed at his farm late last night.

The same sentences rewritten using the active would not be wrong, but the subject of these sentences would sound strange:

You can purchase tickets from the booth at the entrance.
People speak Spanish in much of South America.
Someone killed Edward Moore at his farm late last night.