This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
Sir Ernest Gowers wrote in "The Complete Plain Words" (HMSO,
1954): "The well-known [...] rule against splitting an infinitive
means that nothing must come between 'to' and the infinitive. It is
a bad name, as was pointed out by Jespersen [...] 'because we have
many infinitives without "to", as "I made him go". "To" therefore
is no more an essential part of the infinitive than the definite
article is an essential part of a substantive, and no one would think
of calling "the good man" a split substantive.' It is a bad rule
too; it increases the difficulty of writing clearly [...]." The
split infinitive construction goes back to the 13th century, but was
relatively rare until the 19th. No split infinitives are to be
found in the works of Shakespeare, Spenser, Pope, or Dryden, or in
the King James Version of the Bible.
Fowler wrote (in the article POSITION OF ADVERBS, in MEU) that
"to" + infinitive is "a definitely enough recognized verb-form to
make the clinging together of its parts the natural and normal
thing"; "there is, however, no sacrosanctity about that
arrangement". There are many considerations that should govern
placement of adverbs: there are other sentence elements, he said,
such as the verb and its object, that have a *stronger* affinity for
each other; but only avoidance of the split infinitive "has become
Thus, although in "I quickly hid it", the most natural place for
"quickly" is before "hid", "I am going to hide it quickly" is
slightly more natural than "I am going to quickly hide it". But "I
am going to quickly hide it" is itself preferable to "I am going
quickly to hide it" (splitting "going to" changes the meaning from
indicating futurity to meaning physically moving somewhere), or to
"I am going to hide quickly it" (separation of the verb from its
object). And even separating the verb from its object may become
the preferred place for the adverb if "it" is replaced by a long
noun phrase ("I am going to hide quickly any trace of our ever
having been here").
Phrases consisting of "to be" or "to have" followed by an adverb
and a participle are *not* split infinitives, and constitute the
natural word order. "To generally be accepted" and "to always have
thought" are split infinitives; "to be generally accepted" and "to
have always thought" are not.
Certain kinds of adverbs are characteristically placed before
"to". These include negative and restrictive adverbs: "not" ("To
be, or not to be"), "never", "hardly", "scarcely", "merely", "just";
and conjunctive adverbs: "rather", "preferably", "moreover",
"alternatively". But placing adverbs of manner in this position is
now considered good style only in legal English ("It is his duty
faithfully to execute the provisions...").
Clumsy avoidance of split infinitives often leads to ambiguity:
does "You fail completely to recognise" mean "You completely fail
to recognise", or "You fail to completely recognise"? Ambiguous
split infinitives are much rarer, but do exist: does "to further
cement trade relations" mean "to cement trade relations further",
or "to promote relations with the cement trade"?
The most frequently cited split infinitive is from the opening
voice-over of "Star Trek": "to boldly go where no man has gone
before". ("Star Trek: The Next Generation" had "one" in place of
"man".) Here, "boldly" modifies the entire verb phrase: the
meaning is "to have the boldness that the unprecedentedness of the
destinations requires". If "boldly" were placed after "go", it
would modify only "go", changing the meaning to "to go where no
man has gone before, and by the way, to go there boldly".
Hardly any serious commentator believes that infinitives should
never be split. The dispute is between those who believe that split
infinitives should be avoided when this can be done with no
sacrifice of clarity or naturalness, and those who believe that no
effort whatever should be made to avoid them.