Writers have creative license to build any kind of message they wish. Like architects, they can go wild with ideas and forms. To a certain extent, a writer can bend language to her whim. But like architectural structures, sentences have to obey the laws of physics, or they fall into disorder.
The rule of parallel structures is particularly easy for writers of all levels of experience to miss. It has to do with lists and the reader's expectation, at the start of a sentence, that all the items in a given list will belong together in some way. If that expectation is thwarted, the sentence's structure is marred.
When putting together a list in a sentence, this rule states, you should make sure all the items perform the same basic function in the sentence. Many writers, in their fervor to jot down an idea, end up tossing all sorts of items together in their lists: adjectives bump up against verbs, nouns, verbals, or whatever else the writer conjures up in the moment.
Consider these examples:
1. The supreme leader of the party was distant, contemptuous, and liked to play favorites with his lieutenants.
2. In the lawsuit, the plaintiffs accused the company of polluting the river, the air, degrading the surrounding land, corruption, and malfeasance.
If we split up the first example into its components, we get a beginning and then a list of items that should match that beginning as well as each other:
The supreme leader of the party was
c. liked to play favorites with his lieutenants
When the elements are laid out, it's easy to see that item (c) in the first sentence is the odd one out. It starts with a verb, so it doesn't match (a) and (b), which are both adjectives. Moreover, the verb doesn't refer to the word "was," which touched off the list. We can fix this by regrouping the items, based on the verb they belong to, and linking the two groups with another "and":
The supreme leader of the party [Group 1] was distant and contemptuous and [Group 2] liked to play favorites with his lieutenants.
The sentence is now functionally correct, though the two instances of "and" occurring so close together make it seem leggy and overlong. It might be more elegant to separate item (c) into another sentence, or at least make it an independent clause attached to the first sentence by a semicolon.
The supreme leader of the party was distant and contemptuous; he also liked to play favorites with his lieutenants.
The second example is harder to parse.
In the lawsuit, the plaintiffs accused the company of polluting:
a. the river
b. the air
c. degrading the surrounding land
Here we have an assortment of items and references. While (a), (b), (d), and (e) are all nouns, only (a) and (b) refer back to the gerund "polluting." Item (c), meanwhile, begins with another gerund, and items (d) and (e) don't match either "polluting" or "degrading."
So what is this list about? It should be a list of actions the company was accused of taking. If we start off with the word "of" to form our grouping, we get:
In the lawsuit, the plaintiffs accused the company of:
i. the river
ii. the air
b. degrading the surrounding land
The items don't all begin in exactly the same way, but they all function as nouns, and together they lay out a coherent series of accusations. If we put the whole thing together, we get a sentence that hangs together much better.
In the lawsuit, the plaintiffs accused the company of polluting the river and the air, degrading the surrounding land, corruption, and malfeasance.
Finally, this rule also applies to lists that begin with articles such as "the," "a" and "an." Often, the writer of a list neglects to make sure the items all take the same article or restates a word when it already applies to an item:
Discoveries in this area could be beneficial to the bumblebee, honeybee, and the wasp.
In this case, the final "the" before "wasp" should be deleted, as "the" is already stated at the beginning of the list.
At the beginning of her career, she had been a judge, appellate lawyer, and company president.
For "appellate lawyer," which begins with a vowel sound, "an" is the proper article, so the list needs to state the proper article with each item: a judge, an appellate lawyer, and a company president.
Sorting out sentence parts by their functions isn't always easy, especially when those parts get long and complicated, but it preserves the integrity of the sentence by allowing readers to easily understand how the parts fit together. With these tools, an ambitious writer can build all the way up to the sky without causing the structure to collapse for lack of cohesion.