Why do masculine nouns sometimes change to den instead of der when there isn't a preposition?

Whether the noun begins with den or der depends on the case it is in (and the rule works in the same way for the articles ein/mein/etc). The case is determined by the role the noun is playing in a particular sentence.Mein Bruder ist nett. <- here brother is the subject (nominative = der/mein/ein)Ich habe einen Bruder. <- here brother is the object (accusative = den/meinen/einen)With objects of other genders, the object is also accusative but looks the same as the nominative in the singular.Exception: when the verb 'sein' is used, both the subject and object remain in the nominative, for example: Er ist mein Bruder.