Co-ordinating units of writing must all be of the same grammatical kind. This is what is referred to as preserving parallel structure. For instance, nouns must match with nouns, adjectives with adjectives, prepositions with prepositions, and adverbs with adverbs. In order to make this clear, look at the examples below in which breakdowns in parallelisms are shown.
1. Many ancient beliefs have been rejected as superstitions and detrimental to one's prestige. [noun and adjective]
2. He was a miser, a bachelor, and egotistical. [noun, noun, adjective]
3. John was healthy, wealthy, and an athlete. [adjective, adjective, noun]
4. Celeste was an adult, married, and had a young daughter. [noun, adjective, verbal phrase]
5. The manager praised her employees for their dedication and because they were willing to work on weekends for free. [prepositional phrase, adverb clause]
6. Advertisers not only try to convince viewers that the Kurpotzo is a luxury car but also that the car confers status on its owner. [violation of parallelism with correlative conjunctions]
As you can see, faulty parallelism--or a breakdown in parallel structure--happens when a pair or series of co-ordinating units (nouns, adjectives, adverbs, etc.) are not of the same kind. In number 1, for instance, the writer offers a pair of co-ordinating units, the first a noun, the second an adjective. These are different units, and so the sentence is said to be an instance of faulty parallelism. To correct this sentence the writer might say:
-Many ancient beliefs have been rejected as superstitions and as detriments to one's prestige. [noun and noun]
In number 2, the writer has composed a series of noun, noun, and adjective. A simple correction would be:
- He was a miser, a bachelor, and an egotist. [noun, noun, noun]
Alternatively, this sentence could be corrected this way:
- He was miserly, single, and egotistical. [adjective, adjective, adjective]
The point, of course, is that there can be several ways of correcting faulty parallel structure, although it is usually the case, but not always, that a particular way will be stylistically preferable.
Number 3 is similar to the first two so I will leave it to you to figure out ways of correcting the problems here. In number 4, one might say:
- Celeste was an adult, a married woman, and the mother of a young daughter. [noun, noun, noun]
You now know what you need to do to make your sentences parallel: ensure that the co-ordinating units share a common grammatical structure. For number 5, you can choose whether you want to use two prepositional phrases, or two adverb clauses.
- The manager praised her employees for their dedication and for their willingness to work on weekends for free. [Two prepositional phrases]
- The manager praised her employees because they were dedicated and because they were willing to work on weekends for free. [Two adverb clauses]
Whether or not you can readily identify the parts of speech involved is not as important as developing an ear for detecting when a sentence has lost its parallel structure. In each of the above examples, the sentences simply do not sound right when read aloud. With practice, and even if you can't tell a prepositional phrase from a dangling participle, you can probably fix a sentence once you've determined that there is a problem with its parallel structure.
Finally, the last sentence, number 6, is a different kind of problem in parallelism. This is the problem of a violation of parallelism that sometimes occurs when correlative conjunctions are used: either...or; neither...nor; not (only)...but (also)--these sorts of things. With correlative conjunctions remember this: the same grammatical structure must be on the right-hand side of both conjunctions. Let's look at this sentence by laying it out in two layers to see what this means:
Advertisers not only try to convince viewers that the Kurpotzo is a luxury car
but also that the car confers status on its owner.
On the right-hand side of not only the writer has this grammatical sequence: a verb (convince), a noun (viewers), and a noun clause (that the Kurpotzo is a luxury car). On the right-hand side of but also, the writer has only the noun clause (that the car confers status on its owner). The faulty parallelism can be fixed in either of two ways:
Advertisers not only try to convince viewers that the Kurpotzo is a luxury
car but also try to convince viewers that the car confers status on its owner.
OR (and probably preferable stylistically):
Advertisers try to convince viewers not only that the Kurpotzo is a luxury car
but also that the car confers status on its owner.
Notice that now we have the same grammatical structure on the right-hand side of the two correlative conjunctions. In the first example, following the not only we have "try to convince viewers that…". Following the but also, we have "try to convince viewers that…". These are now the same structure.
In the second version, we have "that the Kurpotzo is…" following the not only, and "that the car confers…" following the but also. Again, we now have a parallel structure.