じゃなかった

じゃなかった is the past negative form of じゃない, which itself is the informal version of the copula だ for nouns and na-adjectives. Use じゃなかった to say something “was not” or “wasn't” in the past. It’s common in casual conversation; in more formal speech, you would use じゃありませんでした or ではありませんでした. Typical contexts include reflecting on past facts, correcting someone, or describing a situation that didn't hold true in the past. Remember that じゃなかった only works with nouns and na-adjectives (not i-adjectives).