sumber: http://jerz.setonhill.edu/writing/grammar/act-pass.htm

Active verbs form more efficient and more powerful sentences than passive verbs

• The subject of an active sentence performs the action of the verb: "I throw the ball."
  
  
• The subject of a passive sentence is still the main character of the sentence, but something else performs the action: "The ball is thrown by me."

1. How to Recognize Active and Passive Sentences

1. Find the subject (the main character of the sentence).
2. Find the main verb (the action that the sentence identifies).
3. Examine the relationship between the subject and main verb.
   • Does the subject perform the action of the main verb? (If so, the sentence is active.)
   • Does the subject sit there while something else -- named or unnamed -- performs an action on it? (If so, the sentence is passive.)
   • Can't tell? If the main verb is a linking verb ("is," "was," "are," "seems (to be)," "becomes" etc.), then the verb functions like an equals sign; there is no action involved -- it merely describes a state of being.

2. Basic Examples

3. Difference between Passive Voice and Past Tense

Many people confuse the passive voice with the past tense. The most common passive constructions also happen to be past tense (e.g. "I've been framed"), but "voice" has to do with who, while "tense" has to do with when.

	Active Voice	Passive Voice
Past Tense	I taught; I learned.	I was (have been) taught [by someone]; It was (has been) learned [by someone].
Present Tense	I teach; I learn.	I am [being] taught [by someone]; It is [being] learned [by someone].
Future Tense	I will teach; I will learn.	I will be taught [by someone]; It will be learned [by someone].

4. Imperatives: Active Commands

A command (or "imperative") is a kind of active sentence, in which "you" (the one being addressed) are being ordered to perform the action. (If you refuse to obey, the sentence is still active.)

• Get to work on time.
• Insert tab A into slot B.
• Take me to your leader.
• Ladies and gentlement, let us consider, for a moment, the effect of the rafting sequences on our understanding of the rest of the novel.

5. Sloppy Passive Constructions

Because passive sentences do not need to identify the performer of an action, they can lead to sloppy or misleading statements (especially in technical writing). Compare how clear and direct these passive sentences become, when they are rephrased as imperative sentences

	To drain the tank, the grill should be removed, or the storage compartment can be flooded.
	Because they do not specify the actors, the passive verbs ("should be removed" and "can be flooded") contribute to the confusing structure of this sentence. Does the sentence 1) offer two different ways to drain the tank ("you may either remove the grill or flood the compartment")? ...or does it 2) warn of an undesirable causal result ("if you drain the tank without removing the grill, the result will be that the storage compartment is flooded")?
	Revision 1: Drain the tank in one of the following ways: remove the grill flood the storage compartment
	Revision 2: 1) Remove the grill. 2) Drain the tank. Warning: If you fail to remove the grill first, you may flood the storage compartment (which is where you are standing right now).

6. Linking Verb: Neither Active nor Passive

When the verb performs the function of an equals sign, the verb is said to be a linking verb. Linking verbs describe no action -- they merely state an existing condition or relationship; hence, they are neither passive nor active.

7. The Passive Voice Is not Wrong

• When you wish to downplay the action:

Mistakes will be made, and lives will be lost; the sad truth is learned anew by each generation.

• When you wish to downplay the actor:

Three grams of reagent 'A' were added to a beaker of 10% saline solution.

(In the scientific world, the actions of a researcher are ideally not supposed to affect the outcome of an experiment; the experiment is supposed to be the same no matter who carries it out. I will leave it to you and your chemistry professor to figure out whether that's actually true, but in the meantime, don't use excessive passive verbs simply to avoid using "I" in a science paper.)

• When the actor is unknown:

  The victim was approached from behind and hit over the head with a salami.

8. Tricky Examples

Punctuality seems important. subject: the phrase "punctuality" action: "being" ("seems" is short for "seems to be") relationship: The subject does nothing at all; the verb "is" functions as an equals sign: "punctuality = important". This sentence describes a state of being (neither active nor passive). (If you replace the single word "punctuality" with the phrase "Getting to work on time" or "The sum total of the knowledge of tribes of prehistoric America collected by amateur archeologists during the latter half of the nineteenth century," the grammar of the sentence does not change.)

Remember to brush your teeth. subject: (You) This is an order; the subject is the person being ordered. action: "remember" (not "brushing") relationship: The subject is supposed to do the remembering. Whether the subject actually obeys the command is irrelevant to the grammar of the sentence. This sentence gives an order. Active. (It may be grammatically possible to give an order with a passive verb, such as a Shakespearean curse like "Be damned!" But most commands you encounter will be active.)