Rule of Parallel Structure
Note: We devote an entire section to parallel structure in the eBook Developing a Powerful Writing Style. We urge you to read it. We also deal with parallel structure in more detail below.
Notice in the examples above that each series contains elements grammatically identical in function and form. If you violate this rule of parallel construction, you will have produced what some grammarians call a shifted construction; others call it a nonparallel construction.
Look at this example:
He likes running and to play soccer.
Here, both elements of the series perform the same grammatical function: direct object of the verb likes. But they appear in different forms, one as an ‑ing verb (present participle) acting as a noun (that is, a gerund), the other as a to verb (infinitive) acting as a noun. To follow the rule, we would write: