じゃなかった is the casual form of
ではなかった. では itself is regularly contracted to じゃ in Japanese, so is worth remembering as its own structure.
じゃなかった (casual), or
ではなかった (semi-polite) are the structures used for creating the negative-past form of nouns and
な-Adjectives. These structures are simply added to the base form of the noun or
な-Adjective.
じゃ and では may also be used with the negative-past ます form of
ある, to create the polite variant.
-
あれは猫(ねこ)ではありませんでした。
That over there was not a cat.
-
あの人(ひと)は綺麗(きれい)ではありませんでした。
That person was not pretty.