I got frustrated by how many of them STILL didn't get it come exam time--what about the dance?? But I, too, remember struggling with passive voice when I was their age. It's not always an easy one to spot, and an even wilier one to try and really understand. So here, my attempt to explain the difference between active and passive voice (and why you should care).
So what do I mean by 'voice', anyway? We're talking about a grammatical category that indicates the relationship between the subject (agent) of your sentence and your verb (action). If you've got an agent carrying out an action in your sentence, then you're using active voice. Verbs are our language's 'doing' words: when an agent in performing what you're describing in your verb, then you're using active voice. If the action of the sentence is happening TO your subject, then you've got passive voice.
Now, for my running man example (minus the dance): when I think of active voice, I think of a man out for a jog at 5AM. He's being ACTIVE - going out and actively carrying out an activity. In order for a sentence to use active voice, it has to do the same thing. Your agent (subject) has to be the one carrying out your verb. For example:
I (agent/subject) once created (action/verb) a fake wedding invitation marrying my brother to one of my friends. (true story - don't ask)
This is what the passive version of this sentence would look like with the subject of the sentence being acted upon.
A fake wedding invitation marrying my brother and one of my friends was created by me.
So we've got a difference in emphasis here. The active voice emphasizes the subject, and the passive voice emphasizes the object or receiver of the action. More examples:
Active: Hope bit her Dad in the leg.Passive: Hope's Dad was bitten in the leg by Hope.Active: Last night I dreamed about that hunky anesthesiologist from the show Offspring.Passive: Last night the hunky doctor from the show Offpring was dreamed about by me.
So here's where my students' eyes start going fuzzy. Because, yes, these sentences are saying the same thing. But they are saying it in different ways, and that's why we care. The passive voice is more difficult fora reader to understand. It's wordier, more roundabout, and often puts space between the actor and action. Sometimes it puts the subject at the end of the sentence so that you don't know who is actually biting Dad's leg until just before the full stop, which can be really confusing. Sometimes the actor doesn't appear in the sentence at all. Passive voice makes for more garbled sentences and, 99% of the time, weaker prose. I mean look at the sentences above: which ones do you prefer? Which ones do you think are easier to read?
It's not that passive voice is always bad. Sometimes you don't want the emphasis to be put on the agent like, say, in a press release from a company that has spilled massive amounts of oil into the sea. They'll say something like "this oversight is regretted", instead of "we regret this oversight." They don't WANT their grammar to sharpen the obvious: that they've done something that people aren't going to be fans of. So it makes sense that they'd want to make themselves out as the object of the action rather than its agent: that's why you see passive voice in so many corporate and government documents. Sometimes the object is more important than the agent. This is the point in the lecture when I'd do my passive man dance, a creepy side-shuffle with jazz hands meant to symbolize someone who is side-stepping the action/blame.
When it comes to writing clearly, active voice is almost always the way you want to go. I've seen so many writers use passive voice without meaning to and then look distraught when they can't figure out how to make their sentence stronger. So when you're reading your work, ask yourself: is the subject of your sentence the one who is doing/has done/will do the action? If not, you're probably using passive voice. And you should probably revise for clarity.
An example from my work:
Passive: A step forward was taken, camera clutched between my hands.Active: I took a step forward, camera clutched between my hands.
... better, right?