The rule against splitting infinitives was imported from Latin by mistake

There is a settled way of talking about the split infinitive rule. It is worth asking how much of it survives contact with the detail.

Where the rule came from

Nineteenth-century writers on English usage frequently modelled their prescriptions on Latin, which was the prestige language of education. A Latin infinitive is a single word, so nothing can be inserted into it, and the question of splitting simply does not arise. Some commentators concluded that the English equivalent should be treated as similarly indivisible, and objected to the construction on that basis.

The objection appears in usage guides from the middle of that century onwards and was adopted enthusiastically by schoolmasters. It is a rule about Latin applied to a language that does not work the way Latin does.

What English writers were doing

Split infinitives appear in English writing for centuries before anyone thought to object, including in respected and formal prose. The construction fell out of fashion for a period and returned, which is normal for stylistic patterns and unrelated to any rule.

No consistent usage tradition was being defended, and the objection was an innovation rather than a preservation. This is the usual finding when a supposedly ancient rule is traced, and it applies to several other famous prohibitions. A rule invented in the nineteenth century to defend a practice that never existed is a peculiar thing to enforce.

When splitting is the clearer option

Placing an adverb between to and the verb often attaches it unambiguously to that verb and nowhere else. Moving it elsewhere can leave genuine ambiguity about what is being modified, which is a real cost.

Tested properly, in some sentences, every unsplit alternative either changes the emphasis or reads as contorted. Writers who follow the rule strictly produce sentences that draw attention to their own avoidance, which is the opposite of good style. The most famous split infinitive in popular culture is a television opening narration, which works precisely because of the split.

What the authorities actually say

Modern usage guides and dictionaries generally treat the construction as fully acceptable, with some recommending it where clarity requires. Even guides that once objected have softened or reversed, describing the rule as unfounded while acknowledging that some readers still dislike it. That last point is practical rather than grammatical, since a reader who believes in the rule will be distracted regardless of its validity.

Writing for an audience that holds a false rule is a rhetorical problem rather than a linguistic one.

The honest position is that the construction is correct and that some readers will nonetheless object.

The pattern behind such rules

Several familiar prohibitions in English usage share this history, having been introduced by individuals and adopted by institutions. They tend to appear in the period when English was being formalised for mass education and needed rules to teach. Rules that can be stated in a sentence and marked in an exercise book have an enormous advantage over rules about clarity.

That selection pressure explains why the surviving prescriptions are so often the arbitrary ones. Actual English usage was described far more accurately by scholars who observed it than by those who legislated for it.

Where the earliest trace is a newspaper anecdote, treat the whole chain with suspicion.

How to decide in practice

Ask what the adverb is modifying and place it where that is unambiguous, which resolves most cases without reference to any rule. Read the alternatives aloud, since the version that sounds forced is usually the one contorted around an invented prohibition.

Consider the audience, because in some formal or conservative contexts the objection still has social force. Never rearrange a sentence into obscurity to satisfy a rule that its own proponents cannot justify. Clarity was always the point, and the rule was only ever a proxy for it that pointed in the wrong direction.

The takeaway

You cannot split a Latin infinitive, and English never asked for the rule that fact produced.

The satisfying version of a story is the one that travels, which is the whole problem.