Split Infinitives
A split infinitive puts a word — almost always an adverb — between 'to' and the base verb it belongs to: to boldly go, to really understand, to quickly finish. It is, famously, one of the great zombie rules of English grammar: a prohibition that was never actually native to English, imported from Latin grammar in the 18th and 19th centuries, and one that virtually every major modern usage authority has since explicitly rejected.
Where the 'rule' against splitting actually came from
In Latin, the infinitive is a single unsplittable word (amare, 'to love'), so it's structurally impossible to insert anything between 'to' and the verb. Some 18th- and 19th-century English grammarians, working from the assumption that Latin grammar represented a more logical or elevated model for English, argued that English infinitives should likewise never be split, since English happens to render its infinitive as two words ('to' plus the verb). This argument mistook a fact about Latin's grammar for a universal law of logic, and it was applied to English despite English never actually needing or historically avoiding split infinitives before these grammarians invented the prohibition.
Why 'to boldly go' became the rule's most famous test case
Star Trek's opening narration — 'to boldly go where no man has gone before' — became, almost by accident, the most widely cited example in the split-infinitive debate, largely because it's instantly recognizable and because 'to go boldly' or 'boldly to go' both sound noticeably weaker and less rhythmic than the split version. Usage commentators have used this line for decades specifically to illustrate that the split form is often the clearly superior stylistic choice, not a mistake to be corrected.
What major usage authorities actually say today
Merriam-Webster, the Chicago Manual of Style, and virtually every serious modern usage guide explicitly describe the split-infinitive prohibition as a myth with no grounding in actual English grammar or historical usage, noting that split infinitives appear throughout English literature for centuries, including in respected classic writers, well before 19th-century grammarians began objecting to the construction. Current guidance uniformly treats splitting an infinitive as a legitimate, often preferable stylistic option rather than an error to be corrected.
When splitting genuinely improves a sentence
Placing the adverb between 'to' and the verb frequently produces the clearest, most natural-sounding placement, especially when moving the adverb elsewhere would create ambiguity or awkwardness: She decided to actually finish the project (clear) versus She decided actually to finish the project (awkward, and arguably changes emphasis) versus She actually decided to finish the project (changes the meaning entirely, now modifying 'decided' rather than 'finish'). In many such cases, the split version isn't just acceptable — it's the version that most precisely places the emphasis where the writer intends it.
When avoiding a split still makes sense
None of this means every split infinitive automatically reads well — inserting a long or multi-word adverbial phrase between 'to' and the verb can genuinely make a sentence clunky: to, after considerable deliberation and much internal debate, finally decide is undeniably awkward regardless of the grammar myth, and most careful writers would move a phrase that long elsewhere in the sentence. The point isn't that splitting is always better, only that whether to split should be decided by what reads most clearly and naturally in a given sentence, not by a blanket rule against the construction itself.
Historical evidence the prohibition was invented, not inherited
Split infinitives appear in English writing going back centuries before the 19th-century grammarians who first objected to them, which is strong evidence the construction was a normal, unremarkable part of English long before anyone declared it wrong. The rule against it appears to have been formalized specifically in 19th-century prescriptive grammar texts, then propagated through school textbooks well into the 20th century, giving it an outsized reputation as a 'real' rule despite having no actual historical basis in how English speakers had used the language for generations before the objection was raised.
How to recognize a zombie rule like this one
Split infinitives belong to a broader category linguists call 'zombie rules': prohibitions that keep circulating in style guides, classrooms, and casual correction long after the grammatical or historical justification behind them has been debunked. Other well-known zombie rules include never ending a sentence with a preposition and never starting a sentence with 'because' — each one shares the same pattern of having been invented or overstated by a small number of influential grammarians, then perpetuated through education long past the point of any real linguistic authority backing it.
Practical guidance for writers today
There is no need to avoid split infinitives on grammatical grounds; the only legitimate consideration is style and clarity in a specific sentence. If splitting the infinitive produces the clearest, most natural phrasing, use it without hesitation. If a particular split feels genuinely awkward — usually because of a long or unwieldy inserted phrase — rephrase for smoothness, but do so because the sentence reads better that way, not because of a mistaken belief that splitting itself is an error.
Split infinitives with more than one adverb
It's also possible, though rarer, to split an infinitive with more than one word: to really truly understand, to completely and utterly reject. These heavier constructions are more likely to feel awkward than a single-adverb split, simply because a longer interruption between 'to' and the verb strains the reader's short-term parsing more than a single word does — this is a genuine stylistic caution, not a resurrection of the old grammatical prohibition, and the same case-by-case clarity test applies regardless of how many words are inserted.
How this zombie rule spread through schools
The prohibition against split infinitives entered widespread American and British education largely through 20th-century school grammar textbooks, which often presented 19th-century prescriptive opinions as settled grammatical fact rather than as one contested position among grammarians. Generations of students were marked down on this basis well after professional linguists and usage authorities had begun rejecting the rule outright, which is part of why the myth still circulates informally today even though it has essentially no support left among serious grammar authorities.
Related myths worth knowing about
The split-infinitive prohibition sits alongside a small cluster of other rules with similarly shaky origins — the idea that a sentence must never end in a preposition, and the idea that 'because' can never begin a sentence, are both discussed in their own dedicated guides, and both share the split infinitive's basic pattern: invented or overstated by particular grammarians, taught as absolute law for generations, and now rejected or heavily qualified by essentially every serious modern usage authority.
A final word on confidence in your own ear
Because the split-infinitive rule has such a long history of being taught as fact, many writers still feel a flicker of guilt inserting a word between 'to' and a verb, even when the resulting sentence reads more naturally that way. Trusting your own ear for clarity and rhythm, rather than an inherited rule with no real grammatical basis, is the better guide here — if the split version communicates your meaning more clearly and reads more naturally, there's no legitimate grammatical reason to avoid it.
One more genuine caution: ambiguity, not error
Occasionally a split infinitive can create genuine ambiguity about which word the adverb is modifying, and in those specific cases repositioning it does improve the sentence: to slowly and carefully review versus to review slowly and carefully might read differently depending on whether the writer wants to emphasize the manner of reviewing or simply attach the description more loosely to the whole clause. This is a legitimate clarity consideration worth weighing case by case, distinct from the debunked blanket prohibition, which applied to every split infinitive regardless of whether ambiguity was actually present.
Frequently asked questions
Is 'to boldly go' grammatically incorrect?
No. It's a perfectly grammatical English sentence, and virtually every major modern usage authority explicitly rejects the idea that splitting an infinitive is an error. The prohibition against it is a 19th-century invention based on mistakenly applying Latin grammar rules to English.
Did any real grammatical tradition ever require unsplit infinitives in English?
No native English grammatical tradition required this; the rule was proposed by a subset of 18th- and 19th-century grammarians reasoning from Latin's structure, not from any evidence about how English itself had actually been used. Split infinitives appear throughout earlier English literature.
Should I still avoid splitting infinitives in formal writing to be safe?
It's rarely necessary, since the rule has been widely debunked in modern usage guides, but a small number of readers and some very conservative style guides still expect unsplit infinitives. If you're writing for an audience or publication you know holds that older view, it's a reasonable accommodation; otherwise, choose whichever phrasing reads most clearly.
Do professional editors still flag split infinitives as errors?
Most contemporary professional editors do not, since major style guides no longer treat it as an error. A minority of more traditionally minded editors or publications may still prefer unsplit infinitives as a house-style choice, but this is now a style preference rather than an enforced grammar rule.