When did split infinitives become poor grammar?

I recall reading an article a few years ago that discussed a period in English when intellectuals began forcing Latin grammar onto the English language, and split infinitives (among other grammatical quirks) became "bad English". My hazy memory is telling me this was some time around the 1700s or 1800s (so this would be unrelated to the Norman Conquest and Middle English).

I also found this stack from English Language Learners that quotes A Brief History of English by Paul Roberts which seems to support the article I'm having trouble remembering:

Another product of the eighteenth century was the invention of "English grammar". As English came to replace Latin as the language of scholarship, it was felt that one should also be able to control and dissect it, parse and analyze it, as one could Latin. What happened in practice was that grammatical description that applied to Latin was removed and superimposed on English. This was silly, because English is an entirely different kind of language, with its own forms and signals and ways of producing meaning. Nevertheless, English grammars on the Latin model were worked out and taught in the schools. In many schools they are still being taught.

Can anyone help me find some specifics on this movement (that is, dates, persons, sources or even quotes)?

That is:

• When did this movement to force Latin grammar onto English gain steam (I'm aware this is something that has been brewing for a long time)?
• Who were some of the main proponents of this movement?

Any other background information is a plus, but answers to those two questions should help me do further research on my own.

Solution 1:

First of all, I'd like to note that few linguists or style-book writers consider split infinitives to be "poor grammar" today, and I don't know if there was ever a consensus among educated speakers against splitting infinitives, so I'm not sure it's accurate to say that they "became" poor grammar.

The origins of this supposed rule seem to be mysterious. Geoff Pullum says that

one source that may have been influential is an 1866 book called A Plea for the Queen's English by Henry Alford (a churchman who was at one time the Dean of Canterbury). Alford gives no evidence or argument at all, but simply cites a prejudice of his own that is contradicted by the testimony of another English speaker who wrote to him. He says (page 188):

238. A correspondent states as his own usage, and defends, the insertion of an adverb between the sign of the infinitive mood and the verb. He gives as an instance, "to scientifically illustrate." But surely this is a practice entirely unknown to English speakers and writers. It seems to me, that we ever regard the to of the infinitive as inseparable from its verb. And when we have already a choice between two forms of expression, "scientifically to illustrate," and "to illustrate scientifically," there seems no good reason for flying in the face of common usage.

I have also heard the "based on Latin" explanation, but I've never seen any direct evidence for it. (I'm aware of the indirect evidence; that the infinitive in Latin is one word and so on.) If anyone has, please downvote this answer and provide the evidence; I'd be very interested in seeing it. Wikipedia also seems to agree with me (for what that's worth; I think the article is at least worth a read).

It seems to be true that Latin grammar played a role in some cases of 18th-century prescriptivism. The following paper gives a bit of an overview: "Prescriptivism and preposition stranding in eighteenth-century prose", by Nuria Yáñez-Bouza. It's about a different topic, preposition stranding, but it still may be of interest. Yáñez-Bouza says that in this time period "Forms not paralleled in Latin grammar were condemned as bad, incorrect, inaccurate, absurd, inelegant, or branded as solecisms." I don't see any direct references to Latin in the quotations she provides from 18th-century critics of stranded prepositions, but the paper cites various sources that might provide more information about the general topic of 18th-century prescriptivism and how it was influenced by Latin grammar. Since you said you are interested in learning about specific figures, you might want to look into James Harris (1751), who Yáñez-Bouza says was "one of the main representative figures in defence of universal grammar and analogy with Latin."

I'm not sure about attributing the "split infinitive" proscription to Latin influence though because early grammarians of English did use other criteria; they didn't just slavishly imitate Latin grammar in all of their descriptions.

According to "Early challengers of norms in the English grammatical tradition," by Henri Le Prieult, it is true that

"the first English grammars — from Bullokar’s Pamphlet for Grammar (1586) to Lane’s Key to the Art of Letters (1700) — [...] display evidence of the passage from the classical tradition of scholarship — when Latin and Greek, often called the instituted languages, had long been the sole tools and objects of learning — to the surprisingly sudden designing and making of grammars of the European vernaculars.

However, according to Le Prieult, "throughout the early modern period, references to this tradition were remarkably bold and critical." Grammarians also made reference to established custom in English, and to principles of "nature and reason." Looking back at the Alford quote, you see that his objection is based on custom (saying the practice is "unknown" to English speakers and not "common usage"); it doesn't rely on Latin grammar in any obvious way.

Solution 2:

The history of split infinitives is laid out in detail on this web page.

A brief summary: they were not split in Old English; most infinitives were a single word, so it actually wasn't possible to split them. And nobody split the remaining ones (beginning with to) either.

In Early Middle English, people started using to to make all infinitives, and not too long after that (early 13th century), people started splitting them.

For unknown reasons, split infinitives stopped being used in Late Middle English and Early Modern English. Shakespeare only used one, where he used an unconventional word order for the sake of a rhyme.

People started using them again in the 18th century. They were pretty rare until the early 19th century, and shortly after that (1834), people started complaining that they were bad grammar—both in the U.S. and the U.K. As is mentioned in the other answer, the first such complaint that received widespread attention was probably Henry Alford's in 1866.

So quite possibly, the reason people complained about split infinitives was not that Latin didn't have them, but that they didn't like people using grammatical constructions that didn't exist when they had learned the English language. People still do that today; for example, read this Language Log post about people complaining about "Can I help who's next?"