Der (dummy pronoun) there (expletive word put in the subject field when the subject is postponed to the predicate field, typically with indefinite subjects or subjectless passive verbs). Der is the masculine nominative article (subject), while den is the masculine accusative article (direct object) and also the dative plural article.
In this video you can learn all existing tips, tricks and hacks how to figure out the gender of each and every noun.. Der der for masculine words, die for feminine words, and das for neuter words.