Answer: 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.
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Some of examples of infinitive verbs:
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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.