This is an irregular verb. It follows the conjugation pattern of the verb llegar.
PresentPresente
La profesora castiga a los alumnos que copian.
The teacher punishes students who copy.
ImperfectPretérito imperfecto
Antes castigaban a los niños sin recreo.
They used to punish children by taking away recess.
Negative ImperativeImperativo negativo
Por favor, no castigues al perro.
Please don't punish the dog.