To is a preposition, meaning "in the direction of" as in Turn to the left, or Come to your senses. It is also an infinitive marker, as in To know you is to love you. Sometimes to in this sense can stand alone, with its verb understood, as in You can do it if you want to.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath
adverb
The part of speech for "explicit" is an adjective.
The part of speech for "lecturer" is a noun.
The part of speech for "rapid" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "breezy" is an adjective.
The part of speech for "buried" is a verb.
The part of speech for "indefinite" is an adjective.