Now notice that last sentence: Jeg har det.. Do you see that et in det? That’s an et because it assumes you are referring to an et word, such as et hus, when you are using the word it. It is kind of the default case.
However, if you were to refer to a thing that is non-neuter (that is, to an en-word), det changes to den. So, how would you say talking about a thing - en ting: