What's wrong with passive voice, you ask? Let me count the ways:
1) Why state something indirectly when you can simply state it? Compare these two sentences:
Passive: The shot was fired by Jim.
Active: Jim fired the shot.
2) Do you want your readers to be confused about who is the subject and who is the object of your sentences? Worse, do you want them still more confused about who does what to whom?
3) Why force your readers to wade through convoluted sentences and needless words? Most of us writers out there have minds that are muddled and convoluted enough as it is. There's no need to advertise it further with bad writing.
4) Passive voice is the written equivalent of up-talking. We are not middle school children, we are writers.
5) Why compromise your own authority? Compare these two sentences:
Passive: It is believed by many that writing is good for the soul.
Active: I believe writing is good for the soul.
Which of these two sentences sounds more authoritative? State your views without equivocation, and don't hide behind others.
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Train yourself to hunt down and kill every instance of passive voice in your writing. Say what you want to say bluntly and directly!