• the subject appears at the end of the sentence in a by-prepositional phrase
• a form of auxiliary be comes in
• the main verb changes into the past participle
• and then you can optionally delete the by-phrase
Here are some examples. Note that the tense is always maintained; passive doesn't affect the tense:
The eagle ate the rat.So those are the logistics. For the most part, we all do it – make active and passive sentences – easily and effortlessly. But sometimes we are advised to avoid the use of passive in writing (though that was it right there). It is viewed (passive again) as obscuring the agent (which it can do, but maybe that’s what you want to do), as being too wordy, or as being too vague (which again, could be the point – that’s why we use it in speech, when we don’t want or don’t need to be specific about who is doing something, about who or what the agent is). In student writing, I often see attempts to avoid passive (or what someone thinks is passive) turn into much more garbled, awkward sentences. If students can easily identify passive, however, then they can make good rhetorical choices about whether they want to use passive or not.
The rat was eaten by eagle.
The police are arresting the suspects.
The suspects are being arrested by the police.
The suggestion to avoid passive is often lumped in with another suggestion to avoid all forms of be. And since forms of be are necessarily a part of the passive verb string, that includes passive. But this avoidance of any form of be can also lead to some bad substitutions when students simply comb through their writing for all forms of be and substitute a not-quite-synonym like exist. Getting students to think about the verbs in their writing is useful. And not using be quite so much is probably a good idea. (It is the most common verb in English, according to the Oxford English Corpus. The is the most common word overall, followed by be.) Some writing instructors note that be (and its forms) does not add enough “umph,” or it simply equates (this is that), or that it’s too general. Considering other verbs and considering some reorganization that leads to the use of other verbs can perhaps improve the writing. But just scanning for be or for passive and then substituting generally makes things worse. It’s really not that time-consuming or difficult to discuss passive and the forms of be and doing so will have other pay-offs since students will then have a clearer understanding of this complex syntactic operation, of the unconscious knowledge they already possess about how it works grammatically and rhetorically, and then, if its use or overuse really is an issue in student writing, students will have the tools to revise.
Another problem with simply telling students to avoid the use of passive is that they’ll see plenty of passive out there in “good writing,” and also their intuitions will tell them that it’s not only not a bad aspect of the language, but can be a very useful one. Here’s a nice list of some of the myths about passive in writing. I will not send you to LanguageLog’s many discussions of the passive (which is mostly about pointing out misidentification of passive - a bit too gotcha - but if you must take a peek, here’s the link).
So recognizing passive is straightforward: a form of be plus the past participial form of a verb. Always. (Ok, I should know better than to say always, but almost always - except when be is get: She got hit by the ball.) And you can check it by creating the corresponding active sentence. Every passive has an active counterpart (though not the other way around since there are plenty of sentences that do not have a passive counterpart. Only sentences that have direct objects can even be passivized.) (You can review past participles in the lesson on the five forms of verbs.)
Passive exercises by Beth Keyser are here, included in a lesson on auxiliary verbs. Much fun will be had by you and your students. :)