1. A good writer uses parallel structure; that is, he or she puts nouns, verbs, phrases, and thoughts into a similar form. This is done primarily for style: it makes the writing easier to read and in turn, easier for the reader to understand. Often, sentences that seem to be correct but just sound wrong have a lack of parallelism at the core of their problem. The ability to write a good parallel sentence is invaluable in essay work. Faulty parallelism, on the other hand, produces an effect in your reader similar to changing gears without using the clutch. A successful parallel sentence reads smoothly, while a faulty parallel sentence lurches awkwardly.
 

Examples of Parallel and Non-Parallel Structure

Nonparallel
Parallel
Students spend their time going to classes, studying, working, and they wish they had time for a social life. Students spend their time going to classes, studying, working, and wishing for a social life.
By the end of the quarter they’re exhausted, irritable, and have learned a lot. By the end of the quarter they’re exhausted, irritable and smarter.
HHS students hope for short school days, and close-in parking spaces are important, too. HHS students hope for short school days and close parking spaces.

2. Parallelism requires that an article (a, an or the) or preposition applying to all members of a series must either appear before the first item only or be repeated before each item.

3. Some words require that certain prepositions precede them. When such words appear in parallel structure, it is important to include all of the appropriate prepositions, since the first one may not apply to the whole series of items.

4. Sentences with correlative expressions (both/and; not/but; not only/but also; either/or; first, second, third) should employ parallel structure as well. Simple rewriting can often remedy errors in these types of sentences.
 

Nonparallel
Parallel
a mark, a yen, buck or pound a mark, a yen, a buck or a pound
on Monday, Wednesday or on Friday on Monday, Wednesday or Friday
(on Monday, on Wednesday or on Friday)
His speech was marked by disagreement and scorn for his opponent’s position. His speech was marked by disagreement with and scorn for his opponent’s position.
a time not for words but action a time not for words but for action
Either you must grant her request or incur her ill will. You must either grant her request or incur her ill will.
My objections are first, the injustice of the measure, and second, that it is unconstitutional. My objections are first, that the measure is unjust, and second, that it is unconstitutional.

Five Rules for Parallelism

  • Use parallel structure with elements joined by coordinating conjunctions. 
  • Use parallel structure with elements in lists or in a series.
  • Use parallel structure with elements being compared.  (X is more than / better than Y)
  • Use parallel structure with elements joined by a linking verb or a verb of being.
  • Use parallel structure with elements joined by a correlative conjunction.

Resources about Parallelism