A split infinitive is a type of language construction common in English that consists of a phrase that begins with the word "to" and ends with a verb in its uninflected form but has an adverb in between. For example, the phrase "to boldly speak" is a split infinitive. Many people consider splitting infinitives a grammar mistake, but this assertion is controversial. Often, grammar books and style guides are silent on the rule or state that it is better to avoid splitting infinitives if possible, but do not specifically state that it is incorrect, and according to some grammar experts, there is no valid rule against it.
When you want to understand what a split infinitive is, it helps to start with being able to recognize infinitives. An infinitive is a verb construction that starts with the word "to" and ends with a verb in its most simple form. Some examples of infinitives include such phrases as "to run," "to love," "to see," "to speak," and "to eat." If the verb is in another form, however, it cannot be a part of an infinitive, as it is incorrect to say or write "to eating" or "to going." A split infinitive is created by inserting an adverb between the word "to" and the uninflected verb.
Sometimes, a person may choose to split an infinitive by inserting another word between the "to" and the verb. This is typically done to make the infinitive more descriptive. For instance, a person may want to say "to go" but also add in the word "boldly," such as in "to boldly go." Adding in the adverb can significantly change the effect of the phrase. For example, "to quickly eat" has a different meaning from "to eat." Often, editors and teachers who consider split infinitives poor grammar constructions prefer to have the adverb tacked on to the end of the phrase instead, such as in "to eat quickly" rather that "to quickly eat."
Though there many sources that state there are no hard-and-fast rules against split infinitives, there are also many grammar experts, teachers, and editors who consider them incorrect, or at the very least, poor choices. For this reason, an individual may do well to avoid them in formal types of writing. Additionally, it is often best to keep the split infinitives simple, such as "to quickly read" rather than "to quickly and cheerfully read."