Passive voice occurs when
the performer of the action is NOT the subject of
the clause (remember that whether dependent or independent, a clause must
have a subject and verb combination).
John was robbed.
In the first case, John did not
the robbing; yet, "John" is the subject of the clause.
John was robbed by a hoodlum.
In the 2nd ex. you see the
obvious: that a "hoodlum" performed the action of robbing John.
As you can see in those two
ex.'s, sometimes you may not see the performer of the action, and sometimes
you will. When you DO see the performer of the action, it will likely
be as an object of the preposition "by."
Something else is far more
obvious about identifying passive voice construction: the presence of a
being verb + the past participle form of a verb.
created, will be manipulated, is explained,
shown, <== note that "show" is an irregular verb) etc.
So watch out, especially for
verbs, to avoid unneccsary use of the passive voice. Passive voice does
not exist with a to be verb + the present participle (that's a verb
passive voice uses just
the past participle.
A good basic formula to remember
be + -ed.
When you realize you're using
passive voice, just create a subject with an appropriate action verb, and
voila! You've got active voice, which in most liberal arts expository writing
creates far more vibrancy and energy.
To create active voice, make
the performer of the action the subject of the clause.
In short: passive voice uses
a "to be" form + "-ed" in a clause wherein the subject is not the performer
of the action.