Learn Hardcore German: Die Bevölkerung in der Stadt ist jung.

Because Bevölkerung is a feminine noun in German. In the nominative singular case (the case used for the subject of a sentence), feminine nouns take the definite article die. So die Bevölkerung literally means “the population.”

Why is in der Stadt dative and not accusative?

The preposition in is a Wechselpräposition (two-way preposition).
• When it indicates location (answers “Where?”), it takes the dative case.
• When it indicates direction or movement (answers “Where to?”), it takes the accusative.
Here we describe a location (“in the city”), so we use dative. Stadt is feminine, and the feminine dative article is der, giving in der Stadt.

Why doesn’t the adjective jung have an ending?

After a copular verb like sein (“to be”), adjectives are used predicatively and remain uninflected. In other words, you say ist jung without adding any adjective ending (no -er, -e, etc.). Predicate adjectives agree in meaning but not in form.

Why is there no comma in Die Bevölkerung in der Stadt ist jung?

German commas separate clauses (especially subordinate or relative clauses) or list items.
This sentence is a single main clause: subject (Die Bevölkerung in der Stadt), verb (ist), predicate (jung). There’s no subordinate clause or enumeration, so no comma is needed.

Can I say in die Stadt here instead of in der Stadt?

No, because in die Stadt uses the accusative and implies movement into the city (“into the city”). Here you’re stating a location (“in the city”), which requires the dative: in der Stadt.

Why is Bevölkerung capitalized?

In German, all nouns are capitalized, regardless of their position in the sentence. Since Bevölkerung is a noun (“population”), it always starts with a capital letter.

Is Bevölkerung always singular, or can it be plural?

Generally, Bevölkerung is a collective noun used in the singular when referring to the population of one place. There is a plural form Bevölkerungen, but you’d only use it when talking about populations of multiple regions or countries (e.g., “die Bevölkerungen Europas”).

Does in der Stadt modify Bevölkerung or the entire sentence?

It’s a prepositional phrase modifying the noun Bevölkerung, specifying which population you mean: the population that is located in the city.

Could I start the sentence with In der Stadt?
For example: In der Stadt ist die Bevölkerung jung?

Yes. German follows the verb-second (V2) rule. If you front In der Stadt for emphasis, the verb ist still occupies the second position: “In der Stadt ist die Bevölkerung jung.”
The meaning stays the same, but you place a bit more focus on the location.

What’s the difference between Bevölkerung and Einwohner?

• Bevölkerung refers to the population as a whole, a collective concept.
• Einwohner (plural) refers to the individual inhabitants or residents.
So die Bevölkerung der Stadt = “the city’s population,” whereas die Einwohner der Stadt = “the city’s inhabitants.”