Parallelism is the idea that parts in a sentence such as items in a series should be phrased in the same grammatical way. The famous quote of Julius Caesar is good example of why parallelism is so powerful.
Parallel: I came; I saw; I conquered.
Here we have three sentences in a row with each of them in the simple past tense and using the same pronoun. It would be a very different quote if parallelism were not present.
Not Parallel: I came; I saw; they were conquered.
The introduction of the new pronoun, they, is jarring and disrupts the flow of ideas.
Smaller parts of sentences should also be parallel.
Not Parallel: I like going to the beach, to go out to eat, and going to the movies.
Clearly, the phrase to go out to eat is not phrased in the same way as the other two. Such inconsistencies jar a reader to questioning credibility or authority of a writer. Good writing requires an even flow.
Parallel: I like going to the beach, going to restaurants, and going to the movies.
Sometimes, it is difficult to see the parts of a sentence that should be parallel. Can you see the nonparallel part of the next sentence?
Not Parallel:
The economic problems were caused by lenders who gave mortgages to people who really could not afford them, by credit cards companies who approved too much easy credit, and by people borrowing more than they could repay.
The economic problems were caused by lenders who gave mortgages to people who really could not afford them, by credit cards companies who approved too much easy credit, and by people borrowing more than they could repay.
Parallel:
The economic problems were caused by lenders who gave mortgages to people who really could not afford them, by credit card companies who approved too much easy credit, and by people who borrowed more than they could repay.
Not Parallel:
John played soccer, but tennis was played by Sonja.
Parallel:
John played soccer, but Sonja played tennis.
Not Parallel:
I like ice cream with chocolate syrup and cake which has butter cream icing on it.
Parallel:
I like ice cream with chocolate syrup and cake with butter cream icing.