What part of speech occurs when an adverb is places between the word to and the verb stem?

Answer 1

"To stem" is an infinitive. When an adverb is placed between "to" and "stem", it creates a split infinitive. A split infinitive is not a part of speech but a highly controversial grammar issue. Star Trek's "to boldly go. . ." is, perhaps, the most famous split infinitive.

Answer 2

This structure is known as a split infinitive. It occurs when an adverb is placed between "to" and the verb stem, such as in the phrase "to quickly run." Split infinitives are often considered grammatically acceptable in modern English, although some traditionalists prefer to avoid them.

Answer 3

It is called a split infinitive, but a split infinitive isn't one of the parts of speech.

Examples of split infinitives:

to quickly run

to generously sprinkle