Learn Hardcore German: Der Fluss fließt ruhig durch die große Stadt. - The river flows calmly through the big city.

The ß in fließt follows the German spelling rule used after a long vowel or diphthong (although there can be nuances). It indicates a specific pronunciation and writing convention in standard German. In Switzerland, you might instead see fliesst with ss, but in most other German-speaking regions, ß is used here.

Why does ruhig appear in this position in the sentence?

Ruhig is an adverb describing how the river flows. In German word order, an adverb often appears after the verb or between the verb and the object (in this case, the phrase durch die große Stadt). Placing it here emphasizes the manner in which the action is happening.

What grammatical case follows durch?

Durch is a preposition that takes the accusative case in German. Therefore, the article and any adjectives referring to the noun after durch must be in the accusative form. Hence die große Stadt is in the accusative.

Why is große spelled with an -e at the end?

Since Stadt is feminine singular and in the accusative case, the adjective groß must be inflected accordingly, becoming große. Adjectives in German change their endings based on the gender, number, and case of the noun they describe.

Could we change the word order to Ruhig fließt der Fluss durch die große Stadt?

Yes, you could place ruhig at the beginning. German word order is relatively flexible, and starting with an adverb can give it emphasis. The main verb typically remains in the second position, so it would be Ruhig fließt der Fluss.... The meaning remains largely the same, though the emphasis changes.