What is Active and passive voice ? What are the rules for voices? Why we use passive voice ?

What is active and passive voice ? What are the rules for voices ? Why we use Passive voice ? 

Active and Passive voice : 

 Voice :   

 Voice is a form of verb that indicate whether the subject does something or something has happened to the subject.The subject does the action or receives the action.

ACTIVE VOICE: 

 A form of voice which states that subject itself is the doer of an action. Its States what the subject does or something is done by the subject.

for example: 

the students take class.

Passive voice: 

 a form of voice which state that an action is done to the subject or it state what happened to the subject or something is done to the subject.

for example:

class is taken by the student.

why passive voice: 

1: we use passive voice for two reason .When the subject is unknown . 

for example : 

Class is taken.

2: when our focus shifts from subject to the object.

 for example:

glass is broken by Ali .

 General rules of passive voice:   

1:subject of active voice become the object of passive voice and object of active voice become the subject of passive voice .

2: be+ Third form of verb past participle is used.

 3: by phrase is used if it needed.

4: sentence with transitive verbs can only be converted into passive voice.

5: been is used for perfect tenses.

6: being is used for progressive tenses