The admissibility of split infinitives has been controversial since the 18th century. Split infinitives are common in English, and have been in use since the thirteenth century. The earliest prohibition of the usage was in 1762, when Robert Lowth argued that because a split infinitive was not permissible in Latin, it should not be permissible in English.
It is worth noting that it is impossible to split an infinitive in Latin, since the Latin infinitive is a single word. Some authorities (e.g. Jesperson and the Merriam-Webster Dictionary of English Usage) claim that the English infinitive is also a single word. The modern English infinitive is derived from the Middle English infinitive, which was marked by the endings -e and -en. The use of to with the infinitive in Middle English almost exactly matched its use in modern English; i.e. sometimes it was used, and sometimes not. See the article on the infinitive for examples of the infinitive without to in modern English.
The split infinitive wasn't used much in the eighteenth century, so Lowth's prohibition didn't attract much attention. Split infinitives became more common in the nineteenth century, and general awareness seems to have started with Henry Alford's condemnation of them in Plea for the Queen's English, published in 1866. By the end of the century, the prohibition was firmly established in the press and popular belief. During certain treaty negotiations with the United States, the British government went so far as to issue instructions to its representatives allowing them to make concessions on fishing rights and reparations, but forbidding them to accept a treaty in which an adverb separated to from an infinitive. The first known use of the term "split infinitive" was in 1897.
However, just as the prohibition against the split infinitive was becoming part of popular culture, there was a reaction against it among leading writers and grammarians. For example, in the 1907 edition of The King's English, the Fowler brothers wrote
There was frequent skirmishing between the splitters and anti-splitters up until the 1960s. George Bernard Shaw wrote letters to newpapers supporting writers who used the split infinitive, and Raymond Chandler complained to his publisher about a proofreader who corrected Chandler's split infinitives.
Even as some grammarians (Alford, cited above; Bache, 1869; Hodgson, 1889) were condemning the split infinitive, others (Brown, 1851; Onions, 1904; Jesperson, 1905; Fowler and Fowler, cited above) were endorsing it. In the present day all reference texts of grammar deem simple split infinitives unobjectionable. (Compound split infinitives still remain controversial.) Nevertheless, a surprising number of teachers and professors of English still admonish students for using split infinitives.
The former prohibition on split infinitives is even more surprising when one observes that there are a number of expressions in English that are weakened considerably by avoiding the split infinitive. The phrase "I plan to really enjoy the party" is more natural and rhythmic than alternatives such as "I plan really to enjoy the party" and "I plan to enjoy really the party". The final possible alternative "I plan to enjoy the party, really" actually possesses a slightly different meaning. (The otherwise perfectly acceptable variation "I really plan to enjoy the party" is not relevant to this particular discussion, as the adverb modifies the indicative verb "plan" rather than the infinitive "enjoy".)
The meaning of other expressions can be changed completely by avoidance of the split infinitive. The sentence "He failed to completely understand the book" suggests that the understanding is not complete, whereas "He failed completely to understand the book" implies that no understanding was achieved at all. Another alternative "He failed to understand the book completely" is ambiguous, as it is not certain whether the adverb is attached to "failed" or to "understand". Finally, the adverb is sometimes placed after the infinitive, as in "He failed to understand completely the book", a construction that can be, according to Fowler, "unnatural".
Split infinitives are also often employed to provide a necessary emphasis in conversation:
Compound split infinitives (where more than one adverb is employed) are still contentious; as recently as 1996 the usage panel of The American HeritageŽ Book of English Usage were evenly divided for and against such sentences as "I expect him to completely and utterly fail." More than three-quarters of the panel rejected "We are seeking a plan to gradually, systematically, and economically relieve the burden." The panel was not entirely consistent however, as 87% deemed "We expect our output to more than double in a year" to be acceptable. Claims that split infinitives are wrong
Counterarguments
The reaction against this "superstition" was based on grammatical, historical, and stylistic considerations. Grammatically, the prohibition of split infinitives was thought to be an application of Latin grammar to a Germanic language. In addition, there are good grounds to argue that to is not part of the infinitive and that placing another word between to and the following infinitive is not splitting the infinitive. Historically, English writers have been separating to from the infinitive at least since Layamon in 1250. And stylistically, the careful placement of another word between to and the infinitive sometimes avoids ambiguity or ugliness. Current views
Problems caused by trying to avoid splitting infinitives
Perhaps the worst problem caused by the belief that infinitives should not be split, however, is that it leads to a wholly groundless avoidance of splitting in other compound verb forms. For example, people will contort sentences to avoid placing an adverb in its correct position between the auxiliary verb and the participle, leading to constructions like, "The argument originally had been used.."(*) instead of the correct, "The argument had originally been used".Compound split infinitives