Sentence elements that are parallel in thought and function must be parallel in form.
Parallel elements in a sentence are two or more words, phrases, or clauses that occur together and have the same grammatical function and the same importance of thought. Parallel structure, when used correctly, creates a symmetrical, graceful construction that is pleasing to the reader. Parallel structure used incorrectly can make writing appear sloppy and can confuse a reader.
1. Three reasons why steel companies keep losing money are that their plants are inefficient, high labor costs, and foreign competition is increasing.
Ineffective: This sentence appears to argue that "their plants are ... high labor costs." Parallel clauses must maintain consistent form.
Better: Three reasons steel companies keep losing money are inefficient plants, high labor costs, and increasing foreign competition.
2. They knew him to be a liar and that he would not keep his appointment.
Ineffective: an and that clause must be preceded by a that clause.
Better: They knew that he was a liar and that... Similarly, an and who clause must be preceded by a who (whose, whom) clause; an and which clause must be preceded by a which clause.]
3. Bushes are shrubs; the Ford is a car, and an example of a beer is Sam Adams.
Ineffective: This sentence illustrates three errors in parallelism.
a. The context here requires parallelism in number: A bush is...a Ford is...a beer is...
b. An example is not logically parallel with a shrub and a car.
c. The punctuation separating the elements in the series is not parallel (a semicolon would imply that the Ford is a car is an extension of the idea that bushes are shrubs).