In a series, each item should take the same grammatical form: all -ing words, all infinitives (to + verb), all nouns, or all clauses. This is called parallel structure.
Key idea
Make every item in a list or series the same kind of grammar. If the first is an -ing word, they all are; if the first is to + verb, they all are.
Why it works
A parallel list has a rhythm the reader can hear, and the matching forms show that the items are equals. A mismatched item trips the reader. Strunk’s fifteenth rule (1918): “Express co-ordinate ideas in similar form.”
Worked example: Infinitives
“The coach told us to stretch, to drink water, and that we should rest.”
- The first two items are infinitives: to stretch, to drink water.
- The third is a clause: that we should rest.
- Make it an infinitive too.
Answer: The coach told us to stretch, to drink water, and to rest.
Worked example: Nouns
“The job needs patience, being careful, and a sense of humor.”
- Patience and a sense of humor are nouns; being careful is an -ing phrase.
- Find a noun for it: care.
Answer: The job needs patience, care, and a sense of humor.
With infinitives you may repeat to or not: “to hike, swim, and ride” and “to hike, to swim, and to ride” are both parallel.
Common mistake
Mixing forms in a list: “to hike, swimming, and a bike ride.” Read the start of the sentence with each item alone: “I like to hike. I like swimming. I like a bike ride.” When the forms change, rewrite to match.