The admissibility of split infinitives has been controversial since the 18th century. Split infinitives are common in English, and have been since before the time of William Shakespeare. 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.)
The first grammarian to argue against this prohibition was Henry Fowler in 1908, and 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.
There are rare examples of non-adverbial insertions into infinitives, as in "It was their nature to all hurt each other."
Splitting infinitives using negations, such as the phrase "I want to not see you any more" are generally considered awkward or ungrammatical, the phrasing "I don't want to see you any more" being preferred.