Here are some examples of how to use it:
* Penal institutions are places where people are incarcerated.
* The penal code outlines the laws and punishments for crimes.
* The judge imposed a penal sentence on the convicted criminal.
You could also use prison as an adjective, though it's less common. For example:
* Prison life is often harsh and isolating.
However, penal is generally the more formal and preferred adjective when referring to things related to prisons.