60-Second Syntax is a quick look at some common mistakes in writing. Please note: Different editors may follow different styles and rules.
Parallelism comes into play when you have words, phrases or dependent clauses in a series. All the words, phrases and dependent clauses in a series should be parallel, meaning they are grammatically equal.
Basically if you are listing items, they should all be the same, whether it be nouns, adjectives, verbs, phrases, gerunds etc. Make sure you verb tenses match.
For example: She studied English, literature and linguistics. (all words in the series are nouns)
She attended Harvard, worked for the Newsweek, and retired from The New York Times. ( a series of past-tense verbs)
First, she had to finish the rough draft; second, she had to get it proofread; and lastly, she had to return it to her professor by Friday ( a series of independent clauses that follow the same structure of introductory phrase, subject, verb; note the verb tense is identical in all sentences.)