Gram.
an expression in which there is a word or phrase, esp. an adverb or adverbial phrase, between to and its accompanying verb form in an infinitive, as in to readily understand.
[ 1895-1900 ]
Usage . The "rule" against placing a word, especially an adverb, between to and the verb in an English infinitive ( To really learn a language, you have to stay in a place where it is spoken ) is based on an analogy with Latin, in which infinitives are only one word and hence cannot be "split." The modeling of English style on Latin has in the past often been considered the epitome of good writing; the injunction against splitting the English infinitive is an example of the misguided application of this notion.
Criticism of the split infinitive was especially strong in 19th-century usage guides. Nothing in the history of the infinitive in English, however, supports the so-called rule, and in many sentences, as in the example above, the only natural place for the modifying adverb is between to and the verb ( To really learn ... ). Many modern speakers and writers depend on their ear for a natural sentence rather than on an arbitrary rule. Writers who ordinarily prefer not to split an infinitive will occasionally do so, to avoid awkward or stilted language.