An infinitive is a base form of the verb plus

An infinitive (i.e., the infinitive form of a verb) is the base form of the verb—PLUS the preposition: "to" in front of the [base form of the verb].

However, the preposition, "to" does not function as a preposition.

Some of examples of infinitive verbs:

1) to be (the base form is: "be"); 
2) to grow (the base form is "grow").
3) to sit;
to stand, to see, to read, to have, to climb, to run, to smile, to climb, to sew, to go, to jump, to think, to swim, to admonish.

Jdjdhdvdhdhsjsjsjsshjssj