An action of a subject, in relation to an object, is expressed in two ways. These ways of expressing an action of a subject are as follows.

1. Active Voice
2. Passive Voice

The structure of a sentence changes when expressed as active voice or passive voice. However, the meaning of the sentence remains the same whether expressed as active voice or passive voice.

To understand Active and Passive voice, we should focus on the subject and the object in a sentence. Almost every sentence has a subject and an object. The subject is the person who does some work. The object is the thing, on which, the work is being done. See the following example.

He is eating an apple.

In the above example, ‘he’ is the subject because he does some work (i.e., eating). In the same sentence, ‘apple’ is an object because some work is being done on it (i.e., it is being eaten). The way this sentence is written (i.e., starts with the ‘subject’ and ends with the ‘object’) is the common style of speaking which is frequently used in our routine. This common style of expression is called Active Voice.

However, there is another way of expressing the action of a subject in relation to an object, in which, the sentence starts with the ‘object’ and ends with the ‘subject’. The sequence of the subject and object is reversed in the sentence. This style of expression is called Passive Voice. For instance, the above sentence can be expressed in passive voice, as follows.

An apple is being eaten by him.

It can be seen in the above example that the places of subject and object are interchanged for converting a sentence from Active Voice to Passive voice. Similarly, it can be said that in active voice, the subject acts upon the object whereas in passive voice, the object is acted upon by the subject. The structure of sentence in active and passive voice can be expressed as follows:

Active voice: Subject + Verb + Object.
Passive voice: Object + Verb + Subject.

Remember, the structure of the sentence changes in active and passive voices, but the meaning the sentence remains the same whether expressed as active voice or passive voice. 

 Rule 1.The structure of the sentence is reversed in passive voice. The places of the subject and the object are interchanged. The subject shifts to the place of the object, and the object comes to the place of the subject in passive voice.

 Rule 2. Change in verb: The 3rd form of verb (e.g., eaten, written) is always used as main verb in passive voice. Whichever form of verb is used in active voice, the verb is changed to 3rd form while converting active voice into passive voice. Remember, only 3rd form of verb is used in passive voice.

 Rule 3. Use of word ‘by’ : The word ‘by’ is always used before the subject in sentences in passive voice.

 Rule 4. Change in helping verb or tense: As noted earlier, only 3rd form of the main verb is used in passive voice. However, the helping verbs (e.g., is, are, was, were, has, had, will) are changed while converting active voice into a passive voice. Since different tenses use different helping verbs, it can be said that the tense of the sentence is changed in passive voice accoring to certain rules which can be studied on this website in detail.

 Rule 5. Expressing subject (pronoun) as object (pronoun): As the subject shifts to the place of the object in passive, the subject pronoun (e.g. he, she, I, they, we) is expressed as object pronoun (e.g., him, her, me, them, us). For instance, we would convert ‘he’ into ‘him’, ‘she’ into ‘her’, ‘I’ into ‘me’, ‘they’ into ‘them’ and ‘we’ into ‘us’. The pronoun ‘you’ remains the same.

 Rule 6. In a very few cases, the subject of the sentence may not be mentioned in the passive voice when its too obvious or needs not to be mentioned.

Note. The above rules, except rule 4, are basic rules for changing active voice into passive and apply to all sentences. The rule 4 is about the usage helping verb in passive voice. Since every tense uses different helping verb (e.g., is, was, had, has, will), the rule No. 4 differs for sentences of different tenses. The rules for changing tense (helping verb) in passive voice can also be studied in detail on this website.