The use of parallels affects the flow of the sentence, and if used correctly they may enable the writer to make a stronger statement through his words by expressing a clear and logical message.
To use parallels is to ensure to use the same forms when writing about two or more ideas. To check for their correct use, skim your writing looking for conjunctions "and" and "or," which are the tell-tale sign of the presence of parallels in a sentence.
Just look at some famous examples of the use of parallelism:
Ask not what your country can do for you; ask what you can do for your country. -John F. Kennedy
We are not satisfied, and we will not be satisfied until justice rolls down like waters and righteousness like a mighty stream. -Rev. Martin Luther King, Jr.
Let's look at two examples of parallelism:
Ex. 1 - My bike is new, pretty and shiny.
Ex. 2 - Mom told me to do my homework, to not stay up late and to brush my teeth before bedtime.
In Ex. 1, we use three adjectives for describing the bike, so that the sentence might flow easier by describing the subject (the bike) this way, instead of, for example, using two adjectives and an infinitive construction: "The bike is new, pretty and goes fast," which doesn´t read as clearly.
In Ex. 2, all three things that Mom said are infinitive constructions, so that the text would flow more easily and be more clear to read than if we said "Mom told me to do my homework, that I should not stay up late, and that I should brush my teeth before bedtime."
When we list things in a sentence, such as in "I had a hamburger, a salad and a coke for lunch," we groups similar items together. Therefore, when listing items of a similar nature, follow a parallel structure to ensure that all items in the list are nouns, adjectives, infinitives, clauses, gerunds or prepositional phrases. In short, all items listed should be in the same grammatical form in order to ensure the clarity of parallel constructions.
The use of prepositions and articles are tricky in parallel constructions. For example, the sentence "I have a snack in the morning, noon and before dinner," is incorrect because the preposition "in" only applies to "the morning" and cannot correctly be applied to "afternoon" and "before dinner." The correct sentence is "I have a snack in the morning, at noon and before dinner"."