1) An infinitive verb is essentially the base form of a verb with the word "to" in front of it. When you use an infinitive verb, the "to" is a part of the verb. It is not acting as a preposition in this case.
Some examples of infinitive verbs include:
to be
to have
to hold
to sleep
to spend
Infinitives are never conjugated with -ed or -ing at the end because they are not used as verbs in a sentence. They’re one of the three types of verbals, which are verbs used as an adjective, noun or adverb. The other verbals are gerunds and participles.
2) A regular verb is any verb whose conjugation follows the typical pattern, or one of the typical patterns, of the language to which it belongs. A verb whose conjugation follows a different pattern is called an irregular verb.
3) Yo - canto
Tú - cantas
El, ella, usted - canta
Nosotros (as) - cantamos
Ustedes - cantan
Ellos (as) - cantan
4) Yo - corro
Tú - corres
El, ella, usted - corre
Nosotros (as) - corremos
Ustedes - corren
Ellos (as) - corren
5) Yo - escribo
Tú - escribes
El, ella, usted - escribe
Nosotros (as) - escribimos
Ustedes - escriben
Ellos (as) - escriben