"The cat affirms it hates the pink fish."
Translation:Con mèo khẳng định nó ghét con cá màu hồng.
Yeah you need the "nó". It's like saying "The cat affirms hates the pink fish" in English. You're combining two phrases to form the complete sentence, and you need to reaffirm the subject. So you need the "it" in English and the "nó" in Vietnamese. I hope that helps. If someone has a better way of explaining it, please go ahead.