Questions & Answers about Der Fluss, den ich sehe, ist breit.
Why is der Fluss marked with der instead of another article?
Because Fluss is a masculine noun. In the singular nominative case (subject of the main clause), the definite article is der.
Why is den used in den ich sehe instead of der?
In the relative clause den ich sehe, den is the masculine singular accusative form of the relative pronoun. It refers back to Fluss as the direct object of sehe.
Why are there commas around den ich sehe?
German grammar requires that relative clauses be set off from the main clause by commas. Here, den ich sehe is a relative clause modifying Der Fluss.
Why does the verb sehe come at the end of the relative clause?
In German subordinate clauses—including relative clauses—the finite verb moves to the final position. Hence ich sehe becomes …den ich sehe.
What determines the word order in the main clause ist breit?