Parallel structure consists of two or more words, phrases, or clauses that are similar in length & grammatical form. For example:
“While on summer break, I went golfing, swimming, and biking.” This sentence has correct parallel structure because each of the verb forms has the same level of importance. This is a balanced sentence.
On the other hand, here is an example of faulty parallelism:
“While on summer break, I learned to golf, swimming, and biking.” The phrase “to golf” is incorrect because it does not match the rest of the sentence.