"An infinitive is a verb in its simplest form, right out of the box. It can usually be recognized by the word to in front of it: Blackbeard helped him to escape. But the to isn't actually part of the infinitive and isn't always necessary: Blackbeard helped him escape. As a preposition, a word that positions other words, the to lets us know an infinitive is coming.

The truth is that the phrase 'split infinitive' is misleading. Since to isn't really part of the infinitive, there's nothing to split. A sentence often sounds better when the to is close to the infinitive: Violet decided to ask for a raise. But there's no harm in separating them by putting a descriptive word or two in between: Violet decided to bravely ask for a raise. Just don't go overboard. Not: Violet decided to for the first time ever and without even blinking ask for a raise.

Writers of English have been merrily 'splitting' infinitives since the 1300s, and it was considered acceptable until the mid-nineteenth century, when grammar books - notably Henry Alford's Plea for the Queen's English - started calling it a crime. (Some linguists trace the taboo to the Victorian's slavish fondness for Latin, a language in which you can't divide an infinitive.) This 'rule' was popular for half a century, until leading grammarians debunked it. But its ghost has proved more durable than Freddy Krueger."

--Woe is I, The Grammarphobe's Guide to Better English in Plain English, by Patricia T. O'Conner