What does it mean to split an infinitive?

A split infinitive is a full infinitive (e.g., “to eat”) that has words (usually adverbs) separating “to” and the verb (e.g., “to noisily eat”).

Split infinitives have traditionally been viewed as ungrammatical, but nowadays they are largely viewed as acceptable and are sometimes necessary.

For example, in “the dentist told Geoff to always brush his teeth before bed,” moving “always” either results in an awkwardly worded sentence or makes it unclear: does “always” refer to how often the dentist tells Geoff or to how often Geoff should brush his teeth?