An infinitive consists of to plus the base form of the verb: to think, to breathe, to dance. When a modifier appears between its two parts, an infinitive is said to be “split”: to carefully balance, to completely understand.
When a long word or a phrase appears between the parts of the infinitive, the result is usually awkward.
Attempts to avoid split infinitives, however, can result in equally awkward sentences. When alternative phrasing sounds unnatural, most experts allow—and even encourage—splitting the infinitive.
At times, neither the split infinitive nor its alternative sounds particularly awkward. In such situations, you may want to unsplit the infinitive, especially in formal writing.
Exercises:
Awkward and misplaced modifiers
Misplaced modifiers 1
Misplaced modifiers 2
Misplaced modifiers 3