What Is Active Voice?
I'll begin with active voice since it is easier to explain and more direct. If a sentence is active, the subject performs the action, and the object, if there is one, is either acted upon or the product of the action. The following are a few simple examples:
Polly cleaned the attic.
Michael borrowed the book.
As you can see, the emphasis is on the performers rather than the recipient.
What Is Passive Voice?
In a passive sentence, the object or product of the action is the subject. Let's look at the sentences above in their passive form:
The attic was cleaned by Polly.
The book was borrowed by Michael.
The focus of these sentences has changed, and the recipients of the action are now in the subject position. But the attic and the book aren't doing anything.
However, it's easy to spot passive voice in simple sentences like the examples. Most writers construct more complex sentences where passive voice tends to hide.
Tracy battled the blustery snowstorm on the way to her stop and waited impatiently to be picked up by the bus.
As you can see, passive voice does not necessarily need to be in the main clause to plague a sentence.
Is Using Passive Voice Always Wrong?
One misunderstanding about passive voice is that it's an error. It isn't. More often than not, the passive sentences I've revised have been fine grammatically. Passive sentences, though, are not normally the best way to express your thoughts. After all, passive voice tends to be awkward and wordy, so you can tighten up your writing by using active voice instead.
However, at times passive voice can have some advantages. For instance, what if party taking action is unknown? Let's look at a common example:
Shots were fired.
Now, this can mean that either the writer knows who fired the shots and is protecting the guilty party or that there were no witnesses and the perpetrator escaped. If you rewrite this as an active sentences, you emphasize an unknown “someone”: Someone fired shots. The same applies if you are deliberately trying to emphasize the recipient or product of the action.
While there are times when passive voice can be useful, keep it to a minimum so your prose will be tight and active.