Parallelism is the expression of similar or related ideas in similar grammatical form. Besides emphasizing the relationships of ideas, parallelism can create intriguing sentence rhythms and highlights. Once you begin a parallel pattern, you need to complete it. If you mix structures, creating incomplete or faulty parallelism, your sentences may disappoint readers' expectations and be hard to read.

Mixed Consider swimming if you want an exercise that aids cardiovascular fitness, develops overall muscle strength, and probably without causing injuries. Parallel Consider swimming if you want an exercise that aids cardiovascular fitness, develops overall muscle strength, and causes few injuries.