A passive construction occurs when you make the object of an action into the subject of a sentence. That is, whoever or whatever is performing the action is not the grammatical subject of the sentence.

Use of Passive

Passive voice is used when the focus is on the action. It is not important or not known, however, who or what is performing the action.
Example: My bike was stolen.


In the example above, the focus is on the fact that my bike was stolen. I do not know, however, who did it.
Sometimes a statement in passive is more polite than active voice, as the following example shows:
Example: A mistake was made.
In this case, I focus on the fact that a mistake was made, but I do not blame anyone (e.g. You have made a mistake.).
 
Form of Passive
 
Subject + finite form of to be + Past Participle (3rd column of irregular verbs)
Example: A letter was written.
When rewriting active sentences in passive voice, note the following:
• the object of the active sentence becomes the subject of the passive sentence
• the finite form of the verb is changed (to be + past participle)
• the subject of the active sentence becomes the object of the passive sentence (or is dropped)

Examples of Passive
 
 Tense - Subject - Verb - Object
Simple Present Active: Rita writes a letter.
Passive: A letter is written by Rita.
Simple Past Active: Rita wrote a letter.
Passive: A letter was written by Rita.
Present Perfect Active: Rita has written a letter.
Passive: A letter has been written by Rita.
Future I Active: Rita will write a letter.
Passive: A letter will be written by Rita.
Hilfsverben Active: Rita can write a letter.
Passive: A letter can be written by Rita.
 
Examples of Passive 

 Tense - Subject - Verb - Object
Present Progressive Active: Rita is writing a letter.
Passive: A letter is being written by Rita.
Past Progressive Active: Rita was writing a letter.
Passive: A letter was being written by Rita.
Past Perfect Active: Rita had written a letter.
Passive: A letter had been written by Rita.
Future II Active: Rita will have written a letter.
Passive: A letter will have been written by Rita.
Conditional I Active: Rita would write a letter.
Passive: A letter would be written by Rita.
Conditional II Active: Rita would have written a letter.
Passive: A letter would have been written by Rita.

Passive Sentences with Two Objects 

Rewriting an active sentence with two objects in passive voice means that one of the two objects becomes the subject, the other one remains an object. Which object to transform into a subject depends on what you want to put the focus on.
 
Subject - Verb - Object 1 - Object 2
Active: Rita wrote a letter to me.
Passive: A letter was written to me by Rita.
Passive: I was written a letter by Rita.
 .
As you can see in the examples, adding by Rita does not sound very elegant. That’s why it is usually dropped.

Personal and Impersonal Passive
Personal Passive simply means that the object of the active sentence becomes the subject of the passive sentence. So every verb that needs an object (transitive verb) can form a personal passive.
 
Example: They build houses. – Houses are built.

Verbs without an object (intransitive verb) normally cannot form a personal passive sentence (as there is no object that can become the subject of the passive sentence). If you want to use an intransitive verb in passive voice, you need an impersonal construction – therefore this passive is called Impersonal Passive.

Example: he says – it is said

Impersonal Passive is not as common in English as in some other languages (e.g. German, Latin). In English, Impersonal Passive is only possible with verbs of perception (e. g. say, think, know).

Example: They say that women live longer than men. – It is said that women live longer than men.

Although Impersonal Passive is possible here, Personal Passive is more common.

Example: They say that women live longer than men. – Women are said to live longer than men.

The subject of the subordinate clause (women) goes to the beginning of the sentence; the verb of perception is put into passive voice. The rest of the sentence is added using an infinitive construction with 'to' (certain auxiliary verbs and that are dropped).
Sometimes the term Personal Passive is used in English lessons if the indirect object of an active sentence is to become the subject of the passive sentence.