When a masculine noun is the subject of a sentence, the definite article will be der. (Nominativ) When a masculine noun is the direct object of a sentence, the definite article will be den. (Accusativ)
Trinkt applies third person singular and second person plural. Trinkst applies to second person singular. You can remember that there's an s in trinkst by Second perSon Singular. Hope that helped!
They are different versions of the word "the", based on what grammatical gender is assigned to the noun they're attached to. You just have to memorize which one to use with with words.
I’m afraid I’m not sure what you’re asking about. Die Kinder is “the children” and der Mann is “the man”. The two words don’t have anything much in common at all.
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