For over two hundred years there has been controversy surrounding split infinitives and their usage in writing.
To clarify, a split infinitive occurs when a word (almost always an adverb) is inserted between the ‘to’ and the verb in a sentence.
Perhaps the most famous example of a split infinitive: “To boldly go (where no man has gone before).” Here, ‘boldly’ is splitting the infinitive ‘to go’. To avoid splitting the infinitive, some have argued it should be written: “to go boldly.”
However, this construction does not have the same prosody or emotional impact – a common occurrence when one avoids using split infinitives.
It has been custom to avoid splitting infinitives (and criticising those that don’t) for almost two centuries. An article dated 1834 called “Inaccuracies of Diction, Grammar” states: “The practice of separating the prefix of the infinitive mode from the verb, by the intervention of an adverb, is not [infrequent] among uneducated persons.”
However, there is little to justify this viewpoint – and plenty to counter it.
For instance, in some cases, it is impossible to avoid split infinitives. These are obligatorily split infinitives, first noted by the American grammarian George O. Curme in 1914.
Furthermore, Geoffrey Pullum, co-author of Cambridge Grammar of the English Language, states: “English doesn’t have an infinitive form of the verb in the way a language like French does. French succéder is a single word, but English to succeed is not; it’s two words. The infinitival subordinating marker to is quite distinct from the verb.”
Avoiding split infinitives can also result in clumsy sentences. For example: “Bob used to secretly keep a diary,” would become: “Bob used secretly to keep a diary,” which sounds awkward.
Alternatively, some split infinitives can hamper the prosody of a sentence. Whether a split infinitive ‘works’ or not is entirely contextual.
British author Norman Lewis states within Word Power Made Easy (1991): “I think the evidence is conclusive enough – it is perfectly correct to consciously split an infinitive whenever such an act increases the strength or clarity of your sentence.”
Ultimately, there is no official grammatical rule within the English Language that dictates split infinitives are incorrect. Those that say otherwise are misguided. You’d be hard-pressed to find a literary master from the last two centuries who haven’t split an infinitive here or there. (Even Shakespeare used a split infinitive: “Root pity in thy heart, that when it grows/ Thy pity may deserve to pitied be.”).