Split Infinitives
An infinitive is the word "to" plus a base verb (e.g., to go, to read). A split infinitive happens when an adverb is placed strictly between "to" and the verb.
Example
She decided to secretly read the letter. (The adverb "secretly" splits "to" and "read").
The Myth
For centuries, strict grammarians argued that splitting an infinitive was a grave error. Today, modern style guides (like APA, MLA, and Chicago) agree that split infinitives are perfectly acceptable, and sometimes preferred.
When to use them
Use them when moving the adverb would create an awkward sentence or change the meaning. (e.g., Star Trek's famous "To boldly go where no man has gone before").