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

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.
 

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

• Towson University
• Purdue University
• Elements of Style Excerpt
• Quiz on Parallelism
• Practice