Splitting Infinitives Has Always Been Fine

To boldly go where no man has gone before.

Somebody, somewhere, has told you that sentence is wrong. They were mistaken, and the mistake has a traceable history.

What a split infinitive is

The infinitive in English is two words: to go, to write, to consider. Put an adverb between them — to boldly go, to quickly write, to carefully consider — and you have split it.

That is the whole offence.

Where the rule came from

In Latin, the infinitive is one word. Ire means to go. You cannot split it, because there is nothing to split — you would have to break the word in half.

In the eighteenth and nineteenth centuries, a number of grammarians took the view that English would be improved by resembling Latin more closely. Latin was the language of scholarship and of a prestigious literature, and English, which is Germanic, was widely felt to be a somewhat disorderly thing that could benefit from tidying. Several rules still taught today come from that project: do not end a sentence with a preposition, do not split an infinitive, use it is I rather than it is me.

The reasoning is circular and was recognised as such almost immediately. English is not Latin. Its infinitive is two words. There is nothing to be gained by pretending otherwise, and the writers who ignored the rule include Chaucer, Donne, Wordsworth, Byron and Henry James.

What every modern authority says

This is not a live dispute. It is settled, and has been for decades:

The Chicago Manual of Style permits it without qualification and notes that avoiding it can produce awkward results.
Merriam-Webster records it as standard and observes that the prohibition never reflected actual usage.
The Associated Press Stylebook permits it where it aids readability.
Fowler’s Modern English Usage, the authority most often invoked by people enforcing the rule, was against enforcing it from the first edition in 1926.

There is no significant modern style guide that forbids it.

Why avoiding it often makes writing worse

The rule is not merely groundless. Following it can change your meaning, which is the serious objection.

Consider:

She failed to completely understand the problem. — she understood it partly.
She failed completely to understand the problem. — she understood none of it, or her failure was total. Genuinely ambiguous.
She failed to understand the problem completely. — probably the first meaning, but it now sounds like an afterthought.

Only the split version says exactly one thing. Moving the adverb to avoid the split does not preserve the sentence; it produces a different, worse sentence and calls it correctness.

And to go boldly where no man has gone before is not the same line. The rhythm collapses and the emphasis lands in the wrong place. Whoever wrote it split the infinitive because splitting it was right.

When not to split

Two situations, neither of them about correctness.

When the split gets long. One adverb is invisible. A phrase is not: to more or less completely and without further delay abandon the plan leaves the reader holding to for far too long before the verb arrives. That is a working-memory problem, not a grammar one, and the same objection applies to any long interruption anywhere in a sentence.

When your reader believes the rule. This is the honest reason to avoid it, and it is not a bad one. If you are writing for someone who will mark it as an error — an examiner, a particular editor, a client with strong opinions — the split infinitive is a fight not worth having in a document that has other work to do.

The wider point

The split infinitive is the clearest example in English of a rule that exists purely because it was asserted confidently for long enough. It was never a description of how the language works. It was a proposal about how the language ought to work, made by people with a particular idea of prestige, and it lost — everywhere except in the folk memory of people who were taught it at school and have never had cause to check.

Which is worth remembering the next time someone corrects your grammar with great confidence. Ask where the rule comes from. Sometimes the answer is a genuine feature of the language. Sometimes it is a nineteenth-century wish that English were Latin.