passive voice

The passive voice is a grammatical construction (specifically, a "voice"). The noun or noun phrase that would be the object of an active sentence (such as Our troops defeated the enemy) appears as the subject of a sentence with passive voice (e.g. The enemy was defeated by our troops).
The subject of a sentence or clause featuring the passive voice typically denotes the recipient of the action (the patient) rather than the performer (the agent). The passive voice in English is formed periphrastically: the usual form uses the auxiliary verb be (or get) together with the past participle of the main verb.

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.

Example passive voice simple present :

1.      a car is designed

2.      the apples are picked

3.      they are cleaned

4.      they are packed

5.      the car is washed

Example passive voice future :

Active: I will write a letter.

1. A letter will be written by me.
   
   
2. She will help me.
   
   
3. I will be helped by her.
   
   
4. John will learn the lesson.
   
   
5. The lesson will be learnt by John.

Example passive voice past :

1. He wrote a letter.
   
   
2. A letter was written by him.
   
   
3. They knew it.
   
   
4. It was known to them.

5. She sang a song.
   
   
6. A song was sung by her.
   
   
7. He loved his friends very much.
   
   
8. His friends were loved very much by him.

Ref : http://www.englishpractice.com/improve/active-passive-voice-simple-tense/

        http://www.englishpractice.com/improve/active-passive-voice-past-tense/

        http://www.englishpractice.com/improve/active-passive-voice-simple present -tense/