Never split an infinitive

What actually happened

The origin is a category error about a different language. In Latin, the infinitive is one word, amare, so it is physically impossible to split. Nineteenth-century grammarians who treated Latin as the perfection of grammar concluded English should not do what Latin could not do, and the rule appeared in schoolbooks from the 1830s onward. English infinitives are two words. There is nothing to protect.

Good writers ignored it continuously. Split infinitives appear in Wycliffe, Chaucer, Donne, Samuel Johnson, Benjamin Franklin, Byron, and Henry James, meaning the construction predates the rule by five centuries. Fowler's Modern English Usage, the most conservative authority available, mocked the prohibition in 1926 and separated "those who neither know nor care" from "those who know and condemn," placing the latter in the wrong.

Avoiding the split often makes the sentence worse or changes the meaning. "He failed to completely understand" and "He failed completely to understand" say different things. Moving the adverb to dodge a fake rule can introduce a real ambiguity, and the Star Trek line is a decent argument all by itself: "to go boldly where no man has gone before" is limp.