"It is a simple hat."
Translation:Det er en simpel hat.
I can understand your confusion, but it should be 'det er en simpel hat'. Even though hat is common gender, and you would say: 'Den simple hat'
In the sentence the hat is not the subject of the sentence but instead what we in Danish call the subject's predicate, and here the gender of the predicate does not necessarily influence the gender of the subject (den/det), at least when it comes to "kopulative" verbs (for instance to be="at være", to be called="at kaldes", to become="at blive" etc)
In Danish, when you describe what something is, we always use 'deT er'. In the same way 'what is it?', is always 'hvad er deT?'
Thank you very much, I believe I understand. Perhaps my confusion comes from the fact that "den" can be used as object when referring to common gender words. Recalling from here: https://www.duolingo.com/skill/da/Pronouns-objective