Parallelism in a Series of Verbs

A list opens a slot, and that slot has one shape. The words that come right before the list set that shape, whether it's a plain verb, a gerund ending in -ing, or an infinitive with 'to.' Once the first item picks a shape, every other item in the list has to fit the same mold. That's why 'She likes hiking, swimming, and to bike' trips you up on the third item: 'hiking' and 'swimming' are both gerunds, so 'to bike' breaks the pattern the first two items already set.

To check a list, read the words that come before it straight into each item, one at a time, like a template. In 'The job requires typing, filing, and to answer phones,' the setup word 'requires' doesn't lock the shape by itself, but the first item in the list does: 'typing' is a gerund. 'Filing' matches it. 'To answer phones' doesn't, so that's the item to fix: it should read 'answering phones.'

This sits next to the rest of sentence structure and formation because it's really about keeping one grammatical identity steady across a clause, the same instinct that keeps tense and voice steady across a whole sentence or essay. A broken list is a small-scale version of a bigger problem: a sentence that starts in one grammatical gear and shifts to another partway through.