Der Hund the dog is ein Hund nominative and einen Hund accusative

In addition to the good explanation of gatiquo.

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You can read the explanation of .....

• the "Three grammatical genders, three types of nouns" in:
  . https://www.duolingo.com/skill/de/Basics-1
  . https://www.duolingo.com/skill/de/The

• German Cases
  . https://www.duolingo.com/skill/de/Accusative-Case