All you ever needed to know about the passive voice you learned in kindergarten, assuming Geoffrey Pullum was your kindergarten teacher*

* and you were precocious and paying attention. This post is for the rest of you.

Let's start with the very practical advice Paul Brians provides in Common Errors in English Usage:

There are legitimate uses for the passive voice: “this absurd regulation was of course written by a committee.” But it’s true that you can make your prose more lively and readable by using the active voice much more often. “The victim was attacked by three men in ski masks” isn’t nearly as striking as “three men in ski masks attacked the victim.” The passive voice is often used to avoid taking responsibility for an action: “my term paper was accidentally deleted” avoids stating the truth: “I accidentally deleted my term paper.” Over-use of passive constructions is irritating, though not necessarily erroneous. But it does lead to real clumsiness when passive constructions get piled on top of each other: “no exception in the no-pets rule was sought to be created so that angora rabbits could be raised in the apartment” can be made clearer by shifting to the active voice: “the landlord refused to make an exception to the no-pets rule to allow Eliza to raise angora rabbits in the apartment.” 

Good advice all the way around, stated succinctly and directly. The bad example sentences are improved by switching to active construction, and the passive verbs are correctly identified ("was written," "was attacked," "was deleted," "was sought," "could be raised").

This seems fairly simple: The passive voice is almost always formed by using a form of "to be" with the past participle form of the main verb. In the previous sentence I did it with "is" and "formed."

And as a usage point, note that Paul does not say it is always wrong or bad style to use the passive—it is the writer's prerogative to judge whether the writing would be improved by changing to active voice. Would my sentence in the previous paragraph be improved by changing it to "One forms the passive by using [. . . ]" or "You form the passive by using [. . . ]"? I decided it would not.
The key is to be well aware of what you are doing as you move along in your writing. Blindly prohibiting the passive voice will not improve your writing, but being aware of this construction and its possible overuse could help.

But is that all there is to know about the passive? Well, hardly. Over at Language Log, Geoff Pullum has been tracking this case for years, and he has thought about the passive voice far more than you or I.

What has Geoff found in all this tracking and thinking? I'll give you some highlights, but keep in mind that I'm burying the lede, which will come at the end. If you wish to skip to the end right this minute, you have to promise me you'll read that document thoroughly. If you plan on clicking the link and skimming for a minute or two, I'm sorry—that will not do. You have to at least get through the salient points:

1. First, there is a shocking amount of misunderstanding about what the passive voice is. You may say, "Well, that's because it's a grammar term and who can be expected to know those?" But Geoff has found one instance after another of "authorities" handing out usage advice without being able to identify a passive. Quick: Name three popular style guides that every writer is familiar with. Chances are you thought of Strunk and White's Elements of Style among those three, but lest you think they know what they're talking about, keep in mind that in that book's discussion of the passive, four example "bad" sentences are provided; but only one of those four is actually a passive. In another instance, Sherry Roberts advises:
   
   "A sentence written in passive voice is the shifty desperado who tries to win the gunfight by shooting the sheriff in the back, stealing his horse, and sneaking out of town."
   
   But Pullum, the grammarian, quickly points out that she herself brazenly uses a passive in composing that sentence (the bare passive adjunct "written in passive voice"). Many other examples of this transgression have been catalogued on Language Log over the years.
   
   
2. Remember that earlier in the post I claimed that the passive is almost always formed by using "to be" with the past participle form of the main verb. True enough, but there are several other passive voice constructions out there: Geoff gives us "I had the suit made by my tailor in Rome," "He got hooked on skeet shooting," and "This rug badly needs washing," along with Sherry Roberts' "A sentence written [. . . ]," as examples of other kinds of passives.
   
   
3. If someone tells you "My first visit to Boston will always be remembered by me" is a passive to be avoided, and that "I will always remember my first visit to Boston" is active and much better, you are entirely within your rights to remind that person that no speaker or writer of English would consider uttering or writing anything so inane as "My first visit to Boston will always be remembered by me." Geoff calls this the "general information-structure constraint," meaning that in a naturally formed passive, there needs to be new information on both sides of the verb, so the phrase "will always be remembered by me" is completely unnatural because the "by me" information adds nothing to the meaning of the sentence. "I will remember" is the natural choice, and trying to twist this sentence around to create a bad example sentence is disingenuous, but indeed, that is what William Strunk did when composing a passive voice example in an early edition of Elements of Style. Yes, you heard me right: "My first visit to Boston will always be remembered by me" was written by Strunk; it is not a real sentence, naturally formed and then later collected for discussion/examination.
   
   
4. What I'm really getting at is that Strunk and White are shifty desperadoes who try to win the gunfight by shooting the sheriff in the back, stealing his horse, and sneaking out of town.
   
   
5. What is labeled as "passive voice" is often just any old phrase or sentence that some critic decides is weak, an utterance that seems designed to shift responsibility and that often but not always contains a form of "to be." Thus "The election results saddened me" and "There was sadness in the room when we heard the election results" have the patina of passive voice, and so a critic ignorant of grammar may deem either of those passive. But they are not. The first sentence is just a verb with a direct object that happens to be "me"; the second sentence uses "There was"—a phrase that often can be revised for the better, but labeling it "passive" is a mistake. Other times sentences are labeled as "passive" for reasons that cannot be discerned or even guessed at. Geoff points out that the Canadian Press Stylebook labels this sentence as passive:
   
   "The economy experienced a quick revival."
   
   It is impossible, for me anyway, to see how anyone could imagine that sentence uses passive voice. Another example from a site called Grammarist calls this a passive:
   
   "I urge him to stay in cable, where he belongs."
   
   I have no idea what "stay in cable" means in this directive (perhaps this is advice to not acquire a satellite dish, or to avoid taking a job at a CBS?) or why "in cable" would be where someone could possibly belong, but I do know that that sentence in no way employs the passive voice.
   
   
6. The passive voice is absolutely required at times; for that reason alone you cannot blindly go about trying to eliminate it. Here's one example from Geoff:
   
   "The picture you’ve been admiring wasn’t painted by Picasso at all; it was painted by me!"
   
   Here the one-to-one symmetry of "by Picasso" and "by me" would be lost entirely were the sentence changed to the active "Picasso didn't paint the picture you've been admiring; I did!" Note the lack of rhetorical punch; note how much worse the active is than the passive in this case. How about the ungrammatical-but-effective exclamation "We was robbed!"? The critique, if there is one, is that the verb does not agree in number with the subject—no one in their right mind would try to "improve" the phrase to "The refs robbed us" or "The judge robbed us." It's important to keep that balance of "we" and "robbed" to emphasize the injustice of the situation.
   
   
7. Regardless of advice you may hear to avoid the passive voice, there is no writer, from the towering literary giant to the casual seller on eBay, who never uses the passive voice (an average of 13–17 percent of the time, in fact). A usage "expert" may advise you to avoid it, but doing so would only make your writing less sophisticated than that expert's.

But enough of the highlights. You really ought to go read Geoff Pullum's "Fear and Loathing of the English Passive." You will learn about several other situations that guide the use of passive voice, and you will be deeply horrified and saddened by the incompetence of many would-be usage guides. Yes, the paper is written for an academic journal, but don't let that discourage you—you won't find a more readable paper on an English usage point, and my summary barely scratches the surface—there are many more juicy parts to discover in the original.