Is the sentence grammatically correct or incorrect. a agustín no puedo creerle nada de lo que dice.

The sentence A Agustín no puedo creerle nada de lo que dice can be translated as I can't believe in anything that Augustine says. So this sentence is a negation because it uses the word no that is used to contradict some or all of the meaning of a positive sentence. Another important thing is the word puedo that is the conjugation of the verb poder for the third person singular in the simple present. Also, le in the word creerle is the indirect object pronoun (third person singular) written at the end of the verb. This is true if and only if the verb is not conjugated, such as infinitives or gerunds or if the verb is an affirmative informal command as the verb creer

me gusta (escribir) en mi cuaderno.

this means : i like to write in my notebook

hopefully this will you out