The principle of parallel structure is to put similar ideas in similar grammatical form.
In this example, note the careful repetition of phrasing in both the subject and the predicate:
Parallel structures emphasize balance. You can balance ideas in pairs, in series, or in lists.
Single words should be balanced with single words, phrases with phrases, clauses with clauses.
Exercises:
Identifying parallel structure
Parallelism 1
Parallelism 2
Parallelism 3
Parallelism 4