Why is it "Ich gehe mit DEM Freund aus" and not "Ich gehe mit DER Freund aus?"

Firstly, great job for remembering that Freund has a masculine article (DER)! Secondly, in German, as you have previously studied, there are 4 different cases (Nominaive, Accusative, Genitive and Dative) and you must remember that articles are inflected differently in every case. I will quickly summarise it with the help of the schema that follows: Nominative   DER (masculine)   DIE (feminine) DAS (neutral) -- DIE (plural) Accusative   DEN (masculine)   DIE (feminine) DAS (neutral) -- DIE (plural) Dative      DEM (masculine)    DER (feminine) DEM (neutral) -- DEN + -n/en (plural) Genitive     DES + -s (masculine) DER (feminine) DES+ -s (neutral) -- DER (plural) In German, specific prepositions go with specific cases, and this sentence, the preposition MIT (with) is always followed by the DATIVE and therefore the article "DER" is inflected into "DEM". Mit is not the only preposition that is linked to the dative, but since there are a few I will teach you a mnemonic so it will be easier for you to remember them: SEMINAVO A UZUBEGE --> SE-MI-NA-VO A U-ZU-BE-GE --> Seit- mit -nach- von -aus -zu -bei -genenüber + DATIVE