In lists
She likes hiking, swimming and to cycle ✗ → hiking, swimming and cycling.
With paired conjunctions
Both … and, either … or, neither … nor and not only … but also should be followed by matching forms: She is not only talented but also hard-working.
In comparisons
Skiing is harder than to snowboard ✗ → Skiing is harder than snowboarding.
In bullet points and headings
Start every bullet with the same kind of word (all verbs, or all nouns). Résumés are the classic place this slips.
Frequently asked
Why does parallelism matter?
Readers expect matching items to look alike. A mismatch makes them re-read, and in formal writing it looks careless.