This is an irregular verb. It follows the conjugation pattern of the verbs cerrar and llegar.
PresentPresente
Él niega su error delante de todos.
He denies his mistake in front of everyone.
PreteritePretérito indefinido
La empresa negó el problema al principio.
The company denied the problem at first.
Present SubjunctivePresente de subjuntivo
Es raro que él niegue la verdad.
It's strange that he denies the truth.