have to eatA casual contraction of 食べなければダメ, literally meaning “if [someone] doesn’t eat, it’s no good.” The subject is understood from context. 食べちゃダメ means “must not eat,” which is a different expression.
JA: 薬を飲む前に、何か食べなきゃダメだよ。
EN: You have to eat something before taking the medicine.