Parallel Structure
Parallel structure means using the same pattern of words to show that two or more ideas have the same level of importance. This can happen at the word, phrase, or clause level. The usual way to join parallel structures is with the use of coordinating conjunctions such as "and" or "or."
Parallel: Mary likes hiking, swimming, and bicycling.
Pamela likes eating, talking, and browsing.
With infinitive phrases:
Parallel: Mary likes to hike, to swim, and to ride a bicycle.
OR
Mary likes to hike, swim, and ride a bicycle.
(Note: You can use "to" before all the verbs in a sentence or only before the first one.)
Do not mix forms.
Example 1
Not Parallel:
Mary likes hiking, swimming, and to ride a bicycle.
Parallel:
Mary likes hiking, swimming, and riding a bicycle.
Example 2
Not Parallel:
The production manager was asked to write his report quickly, accurately, and in a detailed manner.
Parallel:
The production manager was asked to write his report quickly, accurately, and thoroughly.
Example 3
Not Parallel:
The teacher said that he was a poor student because he waited until the last minute to study for the exam, completed his lab problems in a careless manner, and his motivation was low.
Parallel:
The teacher said that he was a poor student because he waited until the last minute to study for the exam, completed his lab problems in a careless manner, and lacked motivation.
Clauses
A parallel structure that begins with clauses must keep on with clauses. Changing to another pattern or changing the voice of the verb (from active to passive or vice versa) will break the parallelism.
Example 1
Not Parallel:
The coach told the players that they should get a lot of sleep, that they should not eat too much, and to do some warm-up exercises before the game.
Parallel:
The coach told the players that they should get a lot of sleep, that they should not eat too much, and that they should do some warm-up exercises before the game.
— or —
Parallel:
The coach told the players that they should get a lot of sleep, not eattoo much, and do some warm-up exercises before the game.
Example 2
Not Parallel:
The salesman expected that he would present his product at the meeting,that there would be time for him to show his slide presentation, andthat questions would be asked by prospective buyers. (passive)
Parallel:
The salesman expected that he would present his product at the meeting,that there would be time for him to show his slide presentation, andthat prospective buyers would ask him questions.
http://owl.english.purdue.edu/owl/resource/623/1/
