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.