What part of speech is combined with the word to form an infinitive?

The part of speech combined with the word "to" to form an infinitive is a verb.

Here's why:

* Infinitive: An infinitive is the base form of a verb, usually preceded by the word "to". It expresses the action or state of being without specifying tense, person, or number.

* Examples:

* to run

* to eat

* to sleep

In each example, "to" is combined with a verb ("run", "eat", "sleep") to create an infinitive.