here are two ways to express an action of a subject in relation to its object

• Active voice
• Passive voice
In “active voice” subject acts upon object, while in “passive voice” object is acted upon by subject. It can also be said, in “active voice” subject does a work on object, while in “passive voice” object is worked on by subject. The normal structure of an active voice sentence is subject+verb+object but in passive the normal structure of sentence is reversed according to certain rules and becomes like object+verb+subject. The real meaning of a sentence does not change if the sentence is expressed either by active voice or by passive voice. The active voice is mostly used in writing because it gives a direct and more concise meaning. Passive voice is used sometimes due to the following reasons.

When intentionally hiding the subject of sentence. For example, a student who failed in exam might say, some chapters were not studied.
Another example. Women were not treated as equals
When passive voice better explain thought of sentence. For example, to say, cloth is sold in yards, is more meaningful than to say, Shop keepers sell cloth in yards.
When passive voice better emphasizes the main though of the sentence. For example, a man who is being teased by another person might say in anger, “you will be beaten by me”
When subject is not exactly known. For example, His watch was stolen. It is not known that who stole his watch, the subject (thief) is not exactly known so it is better to use passive voice for such sentence.There are certain rules for expressing a thought in passive voice or for changing a sentence from active voice to passive voice.
Fundamental Rules for changing from active voice to passive voice

The places of subject and object are interchanged i.e. the object shifts to the place of subject and subject shifts to the place of object in passive voice. Example.
Active voice: I write a letter.
Passive voice: I letter is written by me.
Subject (I) of sentence shifted to the place of object (letter) and object (letter) shifted to the place of subject (I) in passive voice.

Sometimes subject of sentence is not used in passive voice. Subject of sentence can be omitted in passive voice, if without subject it can give enough meaning in passive voice.Examples.
Passive voice: cloth is sold in yards

3rd form of verb (past participle) is always used as main verb in sentences of passive voice for all tenses. Base form of verb or present participle will be never used in passive voice.
The word “by” is used before subject in sentences in passive voice.Example.
Active voice: He sings a song.
Passive voice: A song is sung by him.

The word “by” is not always used before subject in passive voice. Sometimes words “with, to, etc” may also be used before subject in passive voice.
Examples.
Active voice: The water fills the tub.
Passive voice: The tub is filled with water.
Active voice: He knows me.
Passive voice: I am known to him.

Auxiliary verbs are used passive voice according to the tense of sentence.

PASSIVE VOICE FOR ALL TENSES RULES
The places of subject and object in sentence are inter-changed in passive voice.
3rd form of verb (past participle) will be used only (as main verb) in passive voice.
Auxiliary verbs for each tense are given below in the table.

Present Simple Tense (passive Voice)
Auxiliary verb in passive voice: am/is/are
Active voice:
He sings a song.
He does not sing a song.Does he sing a song?
Passive voice:
A song is sung by him.
A song is not sung by him.
Is a song sung by him?

Present Continuous Tense (passive Voice)
Auxiliary verb in passive voice: am being/is being/are being
Active voice:
I am writing a letter
I am not writing a letter.
Am I writing a letter?
Passive voice:
A letter is being written by me.
A letter is not being written by me.
Is a letter being written by me?

Present Perfect Tense (passive Voice)
Auxiliary verb in passive voice: has been/have been
Active voice:
She has finished his work
She has not finished her work.
Has she finished her work?
Passive voice:
Her work has been finished by her.
Her work has not been finished by her.
Has her work been finished by her?

Past Simple Tense (passive Voice)
Auxiliary verb in passive voice: was/were
Active voice:
I killed a snake
I did not kill a snake.
Did I kill a snake?
Passive voice:
A snake was killed by me.
A snake was not killed by me.
Was a snake killed by me?

Past Continuous Tense (Passive Voice)
Auxiliary verb in passive voice: was being/were being
Active voice:
He was driving a car.
He was not driving a car.
Was he driving a car?
Passive voice:
A car was being driven by him.
A car was not being driven by him.
Was a car being driven by him?

Past Perfect Tense (Passive Voice)
Auxiliary verb in passive voice: had been
Active voice:
They had completed the assignment.
They had not completed the assignment.
Had they completed the assignment?
Passive voice:
The assignment had been completed by them.
The assignment had not been complete by them.
Had the assignment been completed by them?

Future Simple Tense (Passive Voice)
Auxiliary verb in passive voice: will be
Active voice:
She will buy a car.
She will not buy a car.
Will she buy a car?
Passive voice:
A car will be bought by her.
A car will not be bought by her.
Will a car be bought by her?

Future Perfect Tense (passive Voice)
Auxiliary verb in passive voice: will have been
Active voice:
You will have started the job.
You will have not started the job.
Will you have started the job?
Passive voice:
The job will have been started by you.
The job will not have been started by you.
Will the job have been started by you?
Note: The following tenses cannot be changed into passive voice.
Present perfect continuous tense
Past perfect continuous tense
Future continuous tense
Future perfect continuous tense
Sentence having Intransitive verbs
Fundamental Rules
The places of subject and object in sentence are inter-changed in passive voice.
3rd form of verb (past participle) will be used only (as main verb) in passive voice.
Auxiliary verbs for each tense are given below in the table.