Grammatically Correct: The writer's essential guide to punctuation ...

Achieving Parallel
Structure
The versatility of the English language makes it possible to express
the same concept in many ways. The problem is, this freedom makes
it all too easy to start composing a sentence one way and then switch
tactics midstream. The result may be a passage that is guilty of what
grammarians call faulty parallelism-a clumsy construction in which
the elements bump up against each other uncomfortably instead of
meshing. In itself each element may be perfectly grammatical, but
it doesn't fit with the others.
Ensuring parallelism in your writing does not mean that every
sentence should be structured the same way. What it does mean is
that if you are creating a sentence, a list or a passage that contains
elements related in purpose or structure, these elements must be
presented in the same grammatical form. Thus, you must not go
arbitrarily from the active voice to the passive, from the second
person to the third, from the present tense to the past, from a series
of adjectives to a noun. You must also be consistent in your use
of minor words such as prepositions, articles, pronouns and
conjunctions.
The effects of faulty parallelism can range from subtle to jarring.
Certain errors may be noticed only on close inspection; others may
render a sentence noticeably awkward or confusing. Some writers
deliberately introduce faulty parallelism out of a mistaken belief that
the variety makes for more interesting reading; more typically, a
writer fails to recognize a problem or simply isn't paying enough
attention. Always remember to focus on the whole as well as the
parts, particularly with sentences that are long and complex.
As with other rules in English grammar, you may at times decide
to bend this one if you feel that a variation in grammatical form
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