Fernando. La diferencia es muy simple. déjame que te muestre ejemplos y te darás cuenta solo. I know the answer = yo sé la respuesta I do know the answer = yo sí sé la respuesta Básicamente, es una confirmación de cuando algo se pone en duda o se quiere confirmar. She loves you = ella te ama She does love you = ella sí te ama Eso es posible en presente y en pasado They went to school = ellos fueron al colegio They did go to school = ellos sí fueron al colegio. En futuro y condicional sólo es posible hacer la aseveración por medio de remarcar el tono sobre el auxiliar ya que el auxiliar no es opcional. A veces en los libros, escriben los auxiliares en cursivas, para que se note el cambio. Espero que esto te ayude.

"I do know" is more emphatic. Example 1. A: The Earth is round. B: I know. (B speaks with no particular emphasis.) Example 2. A. You're so stupid that you don't even know the Earth is round. B. I do know! (Here B is speaking with more emphasis, in order to dispute A's statement that B doesn't know the Earth is round.)

Neil described it perfectly (Mario may have as well, but since I can't read what he wrote I can't say for sure). I just want to add that the literal meaning of the sentence is exactly the same. That is why "Do you...." is used as the question form of many sentences, when 'do' is normally omitted when the sentence is a statement. Using 'do' in a statement like that is only for emphasis, as Neil described.