The previous sentence is an example of good parallelism because it obeys the technique's central rule: The grammatical elements of parallel clauses must match. The following sentence is an example of poor parallelism because the verb form changes:
This is a debate begun in Greece and which continues into modern times.
Begun is a participial adjective while continues is an active verb. The sentence should read:
This debate began in Greece and continues into modern times.
The rule applies not only to verbs but also to nouns, adjectives, adverbs and other parts of speech. In the following sentence, for example, a noun has been mixed with a pair of verbal nouns (gerunds):
I acquired my considerable fortune by investing carefully, hard work and marrying a rich woman.
The sentence should read:
I acquired my considerable fortune by investing carefully, working hard and marrying a rich woman.
Watch for grammatical signposts that point to the need for careful parallel constructions.
"I lost my heart in San Francisco, but I left my shoes in Santa Fe."
Parallel constructions also follow
is better to give than to appear cheap." Here, the to must be repeated to preserve the infinitive structure.
"We can either drive to the Grand Canyon or we're flying to Japan."
The indecisive vacationer should say
"We can either drive to the Grand Canyon or fly to Japan."
Not only . . . but also can be tricky because of the placement of only and also, but the same rules apply;
"He is not only the nicest person I have ever met, but also the most fragrant man I have ever sniffed."
In a not... neither construction, the first negation can affect the meaning of the second. For example, if you write
"Justin is not an ordinary person, and neither are his stories,"
you are implying that the stories are not ordinary people. A more accurate assessment of Justin would be,
"Justin is not ordinary, and neither are his stories."
"I like a good lunch, singing and to read"
"I like eating a good lunch, singing and reading."
Parallelism is especially effective for thesis sentences, because you can incorporate all the sections of your argument in a unified manner:
"Because they are dangerous for children, they stick to carpets, and they are expensive to produce, Slime Balls should be banned from toy stores."
The parallel structure provides a clarity and balance which sharpens your thesis; in this instance the parallels also point the way to the three paragraphs you will be writing to support the thesis.