Similiar to matching, parallelism gives equal weight to words with the same level of importance; using parallelism in a list (three or more things) automatically sounds better to your reader.
- Use with gerunds (-ing): traveling, eating, walking
- Use with infinitive phrases (to, the): to write, to ponder, to revise
- Use with adverbs (-ly): quickly, slowly, craftily
Now compare a sentence that uses parallel structure to one that doesn't:
- Important skills to cultivate in graduate school include thinking, writing, and revising. (parallel!)
- Important skills to cultivate in graduate school include thinking, writing, and to revise. (not parallel.)
The second one feels a little uncomfortable, right? As if you're getting dressed, everything is fitting perfectly, and then you put on your T-shirt backward. The first one, by contrast, is nice and uniform.