The "Rule": Don't split infinitives.
The Reality: Boldly go there.
What is a split infinitive? It's an infinitive used with another word between "to" (the infinitive marker) and the verb itself, and its best-known example is probably the famous Star Trek maxim, "To boldly go where no man has gone before." But no one on the starship Enterprise panicked over a split infinitive, and neither should you.
According to Collins English Dictionary, the traditional rule against placing an adverb between to and its verb is gradually disappearing.
After all, a split infinitive's biggest crime is making a sentence sound clumsy, which does not seem to warrant the absolute condemnation that the practice has attracted. Very often the most natural position of the adverb is between "to" and the verb (as in, "he decided to really try next time") and to change it would result in an artificial and awkward construction ("he decided really to try next time").
The split infinitive has been present in English ever since the 1300s, but it was not until the 1800s that grammarians labeled and condemned the usage. The only rationale for condemning the construction is based on a false analogy with Latin. The thinking is that because the Latin infinitive is a single word, the equivalent English construction should be treated as if it were a single unit. But English is not Latin, and many distinguished writers in history have split infinitives without giving it a thought. So if you choose to start splitting your infinitives, you are in good company, among famed rulebreakers like Benjamin Franklin, Abraham Lincoln, George Eliot and Willa Cather.
Still, those who dislike the construction can usually avoid it without difficulty.
What infinitives are you going to boldly split now that you're allowed?