Passive Voice

Passive Voice:

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 Level: upper intermediate

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.

more info: http://gracielablog-english.blogspot.com/2010/08/passive-voice.html
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