ACTIVE VS PASSIVE
The zombies crossed the road.
The road was crossed by zombies.
A passive voice occurs when the writer makes the object of an action into the subject of a sentence. That is, whoever or whatever is performing the action is not the grammatical subject of the sentence. As you can see above, "zombies" should be the subject of the sentence. When we say "The zombies crossed the road." the subject comes first and actively performs an action. The active sentence is linear from left to right:
>------------------------------>
The zombies crossed the road.
Zombies may be slow, but they're active, and they perform the action.
Unfortunately, some people write in the passive voice, which flips the order of the sentence around. Interestingly, this doesn't necessarily cloud the meaning of the sentence, but it does make it a whole lot less active and a whole lot less interesting to read. I tremble with fear when I'm told, "The zombies crossed the road." but I yawn with boredom when I hear:
<---------------------------------------<
The road was crossed by the zombies.
I don't know about you kids, but I read from left to right, and I'm pretty sure the road didn't move, so why is it in the traditional subject position? The story is being told backward. Lame.
Here are a couple more examples:
>--------------------------------------->
They make Mini-Coopers in that factory.
<------------------------------------------------<
Mini-Coopers are made by them in that factory
>----------------------------------------------->
The sophomores will finish this course by June.
<-------------------------------------------------------<
This course will be finished by the sophomores by June.
>------------------------------------------------------>
Abraham Lincoln killed the zombie with his hatchet.
<-----------------------------------------------------------<
The zombie was killed by Abraham Lincoln with his hatchet.