German marks the ROLE a noun plays in the sentence by changing the article in front of it. Four roles, called cases: Nominativ — who does the action (the subject). Akkusativ — what the action affects directly (the direct object). Dativ — who receives something (the indirect object), and after certain prepositions. Genitiv — who owns something. Mostly written German. The noun itself usually stays the same; the article does the work. Notice that only masculine changes in the accusative — der becomes den. That single change is the most common case error learners make.
Persian marks the definite direct object with «را»: «مرد را میبینم». German has no را at all — it changes the article instead: der Mann → Ich sehe DEN Mann. Same job, different machinery. And Persian has no grammatical gender, so there is nothing in your first language to attach der/die/das to. Learn every noun together with its article from day one — der Tisch, not Tisch.