Using the correct form of the verbs with the expression ababar de…

in the sentence below, why do we use the verb -hablar- instead of hablo?

(1) yo acabo de hablar con mi amigo.
__

(2) what is the error in the two sentences below?

-yo acabo como pizza. __

-ella acabar comprar un lápiz. __

(2) Yo acabó de comer pizza
Ella acaba de de comprar un lápiz

(1) hablar, is an infinitive verb, the verb in its natural form, if we use hablo instead of hablar, in that sentence would be grammatically wrong, 'yo acabo de hablo con mi mejor amigo' that's wrong because 'hablo' is in present an the sentence says that we already talked with our friend.

Hope it help you :)