When you write similar parts of a sentence in the same way, you are maintaining a parallel structure within your writing. Essentially, when a pattern of words are used in a sentence, they should be structured in a way that maintains a clear and concise tone. Incorrectly switching tenses within a sentence or ordering items incorrectly creates an imbalance in sentence structure, which can make it difficult for your reader to understand your point.
A) Carlos enjoys running, painting, and to eat.
B) Carlos enjoys running, painting, and eating.
Because the subject (Carlos) is said to enjoy three activities in a list, those actions must be in the same tense. In example A), the incorrect tense used for the last activity at the end of the sentence disrupts the flow. In example B), all of the actions are in the in the same tense, so the sentence is smooth and easy to understand.
A) Monica was upset about crashing her car and she also felt embarrassed.
B) Monica was upset and embarrassed about crashing her car.
Because the subject (Monica) in example A) has two similar feelings about the same event, those feelings should be listed together, as shown in example B). Doing this cuts down on wordiness and makes the overall point of the sentence easier to follow.
A) Drug side effects may include blurry vision, blindness, or teary eyes.
B) Drug side effects may include teary eyes, blurry vision, or blindness.
Parallelism in writing may also mean listing items in degree of importance. In the above examples, the worst side effect of the drug is obviously blindness, and it should be the last item listed, as shown in example B). Example A) does not give the reader a good indication of the importance of the items on the list, and thus is misleading. This rule also goes for items that fall in a chronological order; for example, you would list the seasons April, May, June, not April, June, May.
A) Dr. Harris is the smartest professor I know; she is also the nicest.
B) Dr. Harris is the smartest and nicest professor I know.
Though example A) is grammatically correct, it is unnecessarily wordy as there is no need for the sentence to be split into two clauses. Combining the adjectives together makes the sentence concise and to the point.
The best way to make sure that your writing is following a parallel structure is to your read work out loud very carefully. Verbally stressing each word in a sentence makes you hear your writing in a new way, and it will be much easier for you to catch your mistakes, especially when it comes to parallelism.