In English, series of two or more items (be they clauses, phrases, or single words) like to be similar. In other words, when lining up multiple items, line up apples with apples, not apples with baseball bats. Aim to express similar ideas in similar grammatical manners. This is what we call parallelism.
You should have parallel structure in three instances:
When using coordinating conjunctions (For, And, Nor, But, Or, Yet, So)
- BAD: People think rats are repulsive because they eat garbage, spread disease, and because they have naked tails. This series goes as follows: verb/noun (eat garbage), verb/noun (spread disease), subordinate clause (because they have naked tails). Do you see that the third item in the series does not sound or look the same?
- BETTER: People think rats are repulsive because they eat garbage, spread disease, and have naked tails. Now we have parallelism! (“Naked” is a necessary adjective and does not wildly throw off the parallelism. However, if one wanted, one could add modifiers in the other two parts of the series: “eat smelly garbage, spread dangerous diseases, and have naked tails.”)
When using correlative conjunctions (not only... but also; neither... nor; either... or; both... and)
Note! For your sentence to be parallel, the correlative and coordinating conjunctions should go directly before the parallel terms.
- ODD: Not only Pierce Brosnan but also Sean Connery, who stars in Indiana Jones and the Last Crusade, plays James Bond.
- BETTER: Not only Pierce Brosnan, who was in The Thomas Crown Affair, but also Sean Connery, who stars in Indiana Jones and the Last Crusade, plays James Bond.
- ODD: I want to go not only to school but also to ride my bike. Here the correlative conjunction is not before the parallel items and it seems strange! The sentence wants to say something about how I wanted to go to school and to another place since the "not only" comes before “school.”
- BETTER: I not only want to go to school, but I also want to ride my bike. OR I want not only to go to school but also to ride my bike. OR I want to go not only to school but also to the movies. All three of these sentences have parallelism, though the parallel terms change depending on where we place the "not only ... but also."
When comparing or contrasting
Implicit in comparisons or contrasts or lists is a need for consistency of subject and verb (tense, number, mood).
- ODD: I do not like apples, pears, or running to the store.
- BETTER: I do not like biking inside, using a Stairmaster, or running to the store. OR I do not like apples, pears, or broccoli. We have changed to the first sentence to a series of gerund phrases, and the sentence is much smoother. We changed the second sentence to a series of nouns.
- ODD: You should have eaten the pie, and I would have eaten the ice cream.
- BETTER: You should have eaten the pie, and I should have eaten the ice cream. OR If you had eaten the pie, I would have eaten the ice cream. The incorrect (“odd”) sentence above has no parallel verbs; the first (“should have”) indicates regret while the second (“would have”) indicates a contrary to fact condition.