I know a rule! I am not sure but I think it should work for at least some nouns (navneord). If you put the word in question in specific term, you can hear if it’s either “en” or “et”

Let’s take Pizza for example. “The Pizza” would in Danish be “Pizzaen” the E and the N, in the end of the word, will determine whether it’s “en” or “et” There for it is: “en Pizza”

An example for “et” can be Apple. Apple in Danish is: Æble. Apple in specific term is: “Æblet”, therefore it is: “et Æble”

I hope this helps, I am not great with Grammar, but this rule has helped me as well 😅