A Split infinitive occurs in English when an adverb is inserted between to and a verb in its infinitive form. An example of the present day is from Star Trek: "...to boldly go where no one has gone before." Here the infinitive verb form of "go" is "to go", and the adverb "boldly" has been inserted within, creating the split.
The admissability of split infinitives has been a source of controversy since the 18th century. Split infinitives are a common aspect of English usage, and have been since before the time of William Shakespeare. The earliest record of someone prohibiting the usage was in 1762 when Robert Lowth forbade them, on the grounds that as 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 dispense with 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). Regardless, 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 an legitimate alternative in terms of this particular discussion, as the adverb has been relocated from "enjoy" to "plan".
Other expressions can have their meaning altered entirely by avoiding the split infinitive. The sentence "He failed to completely understand the book" suggests that the understanding is partial and not complete, whereas "He failed completely to understand the book" implies that no understanding was achieved at all. The other 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".
Split infinitives are also often employed to provide a necessary emphasis in conversation:
- Student A: "I'm going to do better next year." - Student B: "I'm going to really do better next year."
Compound split infinitives (where more than one adverb is employed) are still contentious; as recently as 1996 the usage panel of The American Heritage® Book of English Usage were evenly divided as to the validity of a sentence such as "I expect him to completely and utterly fail." More than three quarters of the usage 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. (See http://www.bartleby.com/64/C001/059.html.)