Active Voice, Passive Voice

There are two special forms for verbs called voice:

1. Active voice
2. Passive voice

The active voice is the "normal" voice. This is the voice that we use most of the time. You are probably already familiar with the active voice. In the active voice, the object receives the action of the verb:

active	subject	verb	object
		>
	Cats	eat	fish.

The passive voice is less usual. In the passive voice, the subject receives the action of the verb:

passive	subject	verb	object
	<
	Fish	are eaten	by cats.

The object of the active verb becomes the subject of the passive verb:

	subject	verb	object
active	Everybody	drinks	water.
passive	Water	is drunk	by everybody.

Source: http://www.englishclub.com/grammar/verbs-voice.htm

Passive Verb Formation

The passive forms of a verb are created by combining a form of the "to be verb" with the past participle of the main verb. Other helping verbs are also sometimes present: "The measure could have been killed in committee." The passive can be used, also, in various tenses. Let's take a look at the passive forms of "design."

Tense	Subject	Auxiliary		Past Participle
		Singular	Plural
Present	The car/cars	is	are	designed.
Present perfect	The car/cars	has been	have been	designed.
Past	The car/cars	was	were	designed.
Past perfect	The car/cars	had been	had been	designed.
Future	The car/cars	will be	will be	designed.
Future perfect	The car/cars	will have been	will have been	designed.
Present progressive	The car/cars	is being	are being	designed.
Past progressive	The car/cars	was being	were being	designed.

Source: http://grammar.ccc.commnet.edu/grammar/passive.htm

Usage

Passive verbs are used when the writer or speaker wishes to focus our attention on the person or thing to which the action is done rather than on the agent - i.e. the person or thing which does the action. Passive verbs also allow the writer to refer to an action without naming the agent. If you want your writing to be concise, direct and informal, you should not use passive verbs too often. The passive voice is more common in formal language and in writing and less common in informal language and in speech.

Online References:
Handout on active and passive verbs - Online Writing Lab

Online exercises:

Some fairly straightforward exercises

A slightly more challenging exercise

A very challenging exercise