"I am not the twenty-sixth, but the twenty-eighth."
Translation:Nem a huszonhatodik vagyok, hanem a huszonnyolcadik.
You're contrasting 26th with 28th. You should put huszonhatodik into the focus. The focus is the spot right before the verb. Your translation would be fine if there wasn't a second part to the sentence.
So when you are comparing two things like 26th and 28th vagyok goes at the end of the sentence as if the sentence were affirmative instead of a negation?