Parallelism occurs when compound verbs or verbals express an action taking place at the same time or in the same tense. When such is the case, the verb and/or verbals must be in the same, or parallel, form.
Gail sings and dances. ("Sings" and "dances" are parallel forms of the verb.)
Types of Parallel Structure
Coordinated ideas of equal rank, connected by "and," "but," "or," or "nor."
Josh is talented not only as a basketball player, but also at playing tennis. (A noun is paired with a gerund.)
Place correlative conjunctions immediately before the parallel terms: