Most English sentences are active, meaning they have a subject that is doing the action. In contrast, a passive sentence has a subject that has the action done to it.
Active: Eli is serving dinner.
Passive: Dinner is being served by Eli.
As these examples show, when you change an active sentence into a passive one, the direct object becomes the subject.
Just like proponents of healthy lifestyles, writing instructors generally encourage (or even require) being active: choosing active verbs instead of passive ones.
Passive sentences are often clunky, and they can be ambiguous if you don’t indicate the agent (the thing that’s doing the action).
Clunky: My mother was visited by me.
No agent: Mistakes were made.
These sentences should definitely be revised (I visited my mother. We made mistakes.), but not all passive sentences are so terrible.
Sometimes the doer (the subject in an active sentence) is not the most important person around.
If you want to emphasize the object, or if the subject is irrelevant or unknown, using the passive voice is better.
Passive: My brother was diagnosed with cancer.
Active: Doctors diagnosed my brother with cancer.
Passive: The toaster should be cleaned monthly.
Active: You should clean the toaster monthly.
Passive: Dominique’s TV was stolen.
Active: Someone stole Dominique’s TV.
With the possible exception of the middle pair (depending on the context), I prefer the passive versions of these sentences. What about you?
Some writing, such as scientific and judicial writing, places a particularly high value on objectivity. By objectivity I mean not using personal pronouns in a lab report and not speculating about who committed a crime.
You can still write active sentences while following these rules, but you might have to write passive ones too, which is perfectly acceptable.
Instructions are another case where you might want to be formal and use passive sentences, or informal and use personal pronouns.
When you write a passive sentence, see if you can change it to an active sentence. If you can easily change it and keep the same meaning and emphasis that it had while passive, make the change.
If changing it results in a clunkier or less accurate sentence, stick with the passive.