Yesterday I got us started on laying the blame for our linguistic insecurity. It was those 18th century grammarians, Robert Lowth and Lindley Murray! They worshipped Greek and Latin and figured the best language would be the language most like those great old Classics. Thus they twisted standard English around the make it seem more Classic. No wonder we make so many grammatical mistakes – our English isn’t sufficiently twisted!
Linguist John McWhorter takes a look at “the odd little idea that we are not supposed to place words between to and a verb, the famous ‘split infinitive.’ This renders sentences like I wanted to carefully explain to her why the decision was made ‘less desirable’ than I wanted to explain to her carefully why the decision was made.” In some constructions prying the adverb from within that ‘infinitive’ leaves the adverb without an unambiguous office in the sentence. “In Latin, infinitives were never split for the simple reason that they were one word, as in most languages.”
How did English’s infinitive come to be two words anyway? “In Old English … an infinitive verb was simply one word, with an ending -an: He began to sing was He ongon singan. To only came to be used with infinitives in Middle English, as endings like -an were shed.” If there’s an explanation for how the word to came to take the place of a word ending, McWhorter doesn’t give it. In fact he seems rather puzzled by the transformation.
source: Word on the Street: debunking the myth of a ‘pure’ standard English by John McWhorter