We human beings seem to have an innate need for order. The deliberate arrangement of things, whether furniture in the living room or ideas on a page, signifies meaning to us. Design, we sense, reveals an intelligence, and that too seems to be an inborn obligation that pleases us when we find it.
One way to bring order to the expression of ideas in writing is to maintain grammatical parallelism. Indeed, to secure this is a major reason for studying grammar. Parallelism means here correspondence, the placing of like grammatical constructions in different but related positions. Take, for example, this sentence a student recently wrote in a paragraph about foreign political dissent:
Protestors fabricated stories on the internet and plastered every wall in the city with posters accusing the police of brutality. Ironically, they were doing exactly what they had accused the government of doing: propaganda.
In the first of these two sentences, the author has set the scene by denoting two actions with transitive verbs (verbs that have a direct object): protestors, he maintains, had fabricated stories and plastered every wall. His next sentence then builds on that idea, and it is cemented nicely to the first sentence by beginning with a reference to what they were doing, pointing once again to action. In fact, as that second sentence continues, the writer reinforces the idea of taking action by repeating the word doing in the prepositional phrase of doing right before the colon.
If we are reading this passage closely, which means in part that we are giving ourselves over to the carry of the sentences, we should be able to sense ourselves stirring mentally a bit from all this movement. The sentences are propelling us somewhere, and our expectation, however unconscious, is that all these actions will lead to yet another. This conclusive momentum, in fact, is the reason the writer rightly felt the need for a colon at the end of the second sentence. A colon presents or unveils or exemplifies, and that is just what we think the writer is about to do: specify the action common between the two contenders. But the bullet does not reach its target. For instead of finding after the colon a verb, the grammatical element that would stand in correspondence with all the previous ones, we find the noun propaganda. The writer did not maintain grammatical parallelism to the very end. This simple slip has taken the propulsive energy out of the passage, and we have fallen before the mark.
In every end, however, is a beginning, and the revision here is a very simple one: propagandizing. This verbal form of the noun will keep the entire passage in parallel, and the reader will sense accordingly a strength and control in idea and expression. The revised second sentence, then, would read: Ironically, they were doing exactly what they had accused the government of doing: propagandizing. The concluding construction after the colon now matches in both form and meaning the like elements that have accumulated across the two sentences leading up to it.
Parallelism is primary in bringing order to both our thinking and writing. For an interesting and informative discussion of the subject (and just about any other topic related to grammar and writing), I would recommend Garner’s Modern American Usage by Bryan A. Garner. A usage manual should sit shoulder to shoulder with a dictionary on the shelf, and Garner’s is an excellent one to always have at hand.