Parallelism is important for the sake of both grammar and style. The principle of parallelism requires the use of the same grammatical constructions in corresponding phrases or clauses.
But what does that mean? The following example illustrates a lack of parallelism:
The article is long, confusing, and an insult to my intelligence.
In this example, the items in a series consist of
(1) an adjective, long, (2) another adjective, confusing, and (3) a noun — insult — plus modifiers.
To make these grammatical elements parallel, change the part of speech in the third item to match the others:
The article is long, confusing, and insulting to my intelligence.
Insulting, like the other items in the series, is an adjective.
A common source of parallelism trouble is the expression as…as. For example:
It is as tall or taller than the Empire State Building.
This is an error because the elements on either side of the coordinating conjunction or are not parallel. You can say, It is…taller than the Empire State Building, but you cannot say, It is as tall…than the Empire State Building. To correct the error, add another as to the sentence:
It is as tall as or taller than the Empire State Building.
The revised version is the correct shorthand equivalent of writing, It is as tall as the Empire State Building, or it is taller than the Empire State Building.
A final example of a parallelism problem involves the frequently misused correlative conjunction not only…but also:
The book is not only boring but also costs a lot.
Whenever you use not only…but also, the grammatical elements linked by the expression must be the same — for instance, two nouns, two predicates, and so on. In the example above, boring is an adjective. Therefore, what follows but also should also be an adjective. In the sentence’s current form, however, what follows is instead a verb: costs.
To fix the error, write:
The book is not only boring but also expensive.