Questions & Answers about Die Stadt ist bekannt für den Fluss.
Because the preposition für always takes the accusative case in German.
- der Fluss = nominative (subject form)
- den Fluss = accusative (direct object form, masculine singular)
In the sentence Die Stadt ist bekannt für den Fluss, den Fluss is the object of the preposition für, so you must use the accusative form den, not the nominative der.
Die Stadt is in the nominative case, because it is the subject of the sentence, the thing that “is known.”
- Stadt is a feminine noun.
- The feminine definite article in the nominative is die.
So:
- die Stadt (feminine, nominative, subject)
- den Fluss (masculine, accusative, object of für)