Here's why:
* Adverbs can be placed in various positions within a sentence: They can come before the verb, after the verb, or even at the beginning or end of a sentence. There's no fixed rule about their placement between "to" and a verb stem.
* The structure you're describing isn't a distinct grammatical construction: It's simply an example of adverb placement within a sentence, and the specific rules for adverb placement vary depending on the language and the type of adverb.
However, the specific structure you described might be relevant in certain contexts:
* Infinitive phrases: In some cases, an adverb placed between "to" and the verb stem might be part of an infinitive phrase. For example, in "He decided to quickly leave," "quickly" modifies the infinitive "leave."
* Specific grammatical rules in certain languages: Some languages might have specific rules for adverb placement within infinitive phrases.
To understand the grammatical function of the adverb in your specific example, you would need to provide more context. Please share the full sentence or phrase you have in mind.