When parts of a sentence are equivalent, they should be expressed in parallel form. Parallelism can refer to single words, phrases, or clauses. If you have trouble with parallelism, simply go through one element at a time and figure out what form each is in. You can then revise any that don't mesh with the whole.
NOT PARALLEL: He is honest, you can trust him, and so kind.
This list gets confusing because the elements aren't grammatically parallel. If you revise to make all the elements into adjectives, the sentence sounds much better:
PARALLEL: He is honest, trustworthy, and kind.
Sometimes elements are much more complicated than single words, so parallelism becomes even more important for clarity:
PARALLEL: As I've gained experience in the working world, I've realized that it's important to be pleasant to your coworkers, to take criticism kindly, to always be on time, and to get your work done promptly.
Each item in this series is expressed in the infinitive form, making the whole list parallel and, therefore, easier to understand.
Often parallelism is forgotten in sentences that contain pairs of things. Consider the following example:
NOT PARALLEL: In my high school, bad grades could be grounds for detention or being suspended.
Detention and being suspended aren't parallel. Revise as follows:
PARALLEL: In my high school, bad grades could be grounds for detention or suspension.
The same problem can occur in comparisons:
NOT PARALLEL: Flying off the handle is easier than to stay calm.
The two elements in this sentence are not working together because of the lack of parallel construction. Revise as follows:
PARALLEL: Flying off the handle is easier than staying calm.
Often function words6 can be repeated at the beginning of parallel structures that are complicated or easily missed when reading.
UNCLEAR: My doctor told me that I needed to make a lot of changes to my lifestyle but I needed to introduce them gradually.
Adding a second that can help this sentence read more clearly:
REVISED: My doctor told me that I needed to make a lot of changes to my lifestyle but that I needed to introduce them gradually.
6 Usually prepositions (in, for, with, etc.) or subordinating conjunctions (that, because, etc.) that "signal the grammatical nature of the word groups to follow" (Hacker 113).