Here's a handout about passive voice--it's not meant for this particular audience, so for heaven's sake don't feel like I'm dumbing down. It's not really designed for y'all.
Verbs are words of action (run, jump, think, speak, eat, laugh, love, sing).
Voice describes the form a verb takes depending on whether the subject of the verb acts or is acted upon. If the subject of the verb performs the action, the verb is in active voice. If the subject of the verb receives the action, then the verb is in passive voice.
Active voice: Jane Austen wrote Northanger Abbey.
Passive voice: Northanger Abbey was written by Jane Austen.
Normally the person or thing who performs the action of the verb is a noun or noun phrase. In a sentence that uses passive voice, you can't tell who or what specifically performed the action of the verb. You just know that it happened. The subject may be buried in a prepositional phrase (by Jane Austen)
Passive voice: The ball was kicked.
• You can't tell who “did” the kicking.
Active voice: John kicked the ball.
• John “did” the “kicking.”
John is the grammatical subject of the sentence.
You can think of passive voice as an equation, or a recipe, if you like:
Passive voice = was, is, being, or has been + a verb in the past tense
• You always have a form of “be” in a sentence in passive voice (was, is, being, or has been).
• The be verb is always followed by a verb in the past tense.
• You can't tell from the sentence who “did” the action of the verb.
Passive voice, in and of itself, is not evil. It's often very useful, particularly in writing in the sciences or social sciences, where quite often the writer truly doesn't know who is responsible for an action.
• You might want to use passive voice to emphasize an action or the recipient of that action, rather than the “doer.”
Driving tests are given to all employees.
• Sometimes you want to “soften” bad news.
Your watch can not be repaired under warranty.
• Sometimes you want to avoid responsibility.
An error was made on your tax return.
But in other sorts of writing passive voice sounds like you are not sure of yourself, and thus weakens your argument. Unless you have a good reason to use passive voice, avoid it. Revise sentences in which you do not explicitly state who is doing what to whom so that you have strong declarative statements built around an active verb.