Parallel construction results when two or more related phrases are in the same grammatical form; for example, they’re all nouns, they’re all verbs, or they’re all gerunds (verbs ending in –ing that act as nouns). When items aren’t parallel, a sentence sounds awkward, as in the following example:
The company rewarded its employees for their hard work, expertise, and paying attention to detail.
Even to the untrained ear, something sounds a little off about that sentence. The terms hard work and expertise are standard nouns, but the last item, paying attention to detail, is a gerund. Parallelism clarifies the meaning and improves the sound of the sentence:
The company rewarded its employees for their hard work, expertise, and attention to detail.