I need some bit of a reminder. That is to say... I used to know these things, but forgot about them somewhere in life. Why is that certain "diptongos" have to be "dissolved" with a tilde? I'm talking about "economía", "cafetería" and others, while "puerta", "periodo" and "hacia" do not need a tilde? Is is only because of philological reasons? That is to say, because these words are pronounced that way? Please, advice... I must reply to a question from a student. Gracias desde ya!