Parallel structure occurs when a sentence pattern contains a series (I bought milk, turkey, and cheese.) The key to parallel structure is that all the parts in the series should match up in form.
For example: Craig wrote his essay, washed his dog, and put away his laundry.
This example works as a parallel structure sentence because each part of the series is parallel (all three verbs line up: wrote, washed, and put). Also, all three nouns line up: essay, dog, and laundry. The fact that all three parts are parallel conveys that all three components are equally important. Another important point is that parallel structure illustrates uniformity and balance, which makes for a pleasant reading experience.
Now let’s look at an incorrect example of parallel structure. Knowing the wrong way to use parallel structure will help you to avoid making a similar mistake:
The doctor showed Angie how to lower her blood pressure, what it takes to manage her cholesterol, alleviate her headaches, remove warts, and even anxiety.
Since this sentence does not contain parallel structure, there is no logical flow to the sentence, and it is difficult for readers to pick out the important parts of the sentence or how the different parts of the series are related. Form follows function: when the grammatical structures of the sentence line up (when parallel structure is used correctly), the logical meaning of the sentence is more likely to make sense as well.
Here’s an easy fix: To help make this sentence fit parallel structure, try picking out the verb that seems to fit the most nouns in the sentence and change those places to make the sentence follow parallel structure. You can also change the other verbs into verb infinitives (add the word “to” before each verb) to help each part of the series become parallel.
Let’s look at it again: The doctor helped Angie to lower her blood pressure, to manage her cholesterol, to alleviate her headaches, to remove her warts, and to relieve her anxiety.
If a sentence breaks the rules of parallel structure and if making it fit parallel structure makes the sentence too bulky or become too repetitive, ask yourself if the ideas you are trying to make parallel truly belong in parallel structure or if the sentence can work more effectively in some different way.