Hola Juliet. If you want to say in Spanish: You understood it wrong. The correct way is: Tú entendiste mal. Entendido is a past paticiple. Example: Tú has entendido mal. (You have understood wrong.)

Hi Juliet! In Spain we would say: "Lo entendiste mal", "Lo has entendido mal", "No lo has entendido bien" (lo = it). If you want to be more polite, you can say: "Lo entendió mal", "Lo ha entendido mal", "No lo ha entendido bien"

Entendiste mal. Has entendido mal. No entendiste bien. No has entendido bien.