Say something did not happen (plain past)
なかった is the plain past negative of a verb, saying it did not happen.
JLPTN5
[verb negative stem] + なかった~ nakatta
did not [verb] (casual speech)
なかった is the plain (casual) past negative of a verb, made by changing the ない form's final い to かった. It means "did not (do)" and is the form to use with people you are close to. In polite speech you would use ませんでした instead.
Conjugation
食べる→食べなかった
ichidan
飲む→飲まなかった
godan
行く→行かなかった
godan
する→しなかった
irregular