Learn Hardcore German: Die Feuerwehr kommt schnell, wenn ein Notfall gemeldet wird. - The fire department arrives quickly when an emergency is reported.

Because Feuerwehr is a collective noun in German that is grammatically feminine singular. Even when it denotes the entire fire department (a group of individuals), it still takes die and behaves like a singular noun.

What part of speech is schnell, and why does it come right after the verb kommt?

Here schnell is an adverb modifying kommt. In German main clauses, the finite verb must occupy the second position (the V2 rule), so adverbs typically follow immediately after the verb.

Why is there a comma before wenn?

wenn introduces a subordinate clause. In German, you must separate main and subordinate clauses with a comma.

Why does wird appear at the end of the subordinate clause wenn ein Notfall gemeldet wird?

In a subordinate clause, the finite verb (here wird) moves to the very end of that clause.

What construction is gemeldet wird, and how does it differ from an active sentence?

gemeldet wird is the present-tense passive voice of melden. The focus is on the emergency being reported, not on who reports it. An active equivalent would be jemand meldet einen Notfall, where jemand is the subject performing the action.

Why is ein Notfall in the nominative case instead of the accusative after wenn?

In the passive construction, ein Notfall functions as the subject of wird gemeldet. Subjects in German always appear in the nominative case, even inside subordinate clauses.

Why is the indefinite article ein used in wenn ein Notfall gemeldet wird? Could you use der instead?

Using ein Notfall makes it a general statement about any emergency. If you said wenn der Notfall gemeldet wird, you’d refer to a specific, previously mentioned emergency. The indefinite article is typical for generic contexts.

Does wenn here mean “when” in the sense of time, or “if” as in a condition?

In this sentence, wenn has a general temporal meaning (“whenever/when an emergency is reported”). German uses wenn for both conditional (“if”) and temporal (“when/whenever”) clauses; context clarifies which sense is intended.

Why isn’t there a direct object in Die Feuerwehr kommt schnell?

The verb kommen is intransitive and does not take a direct object. Die Feuerwehr is the subject, and schnell is an adverbial modifier.

Can you move schnell to the front of the sentence for emphasis? What changes?

Yes. If you front schnell, you must still obey the V2 rule by inverting subject and verb:
Schnell kommt die Feuerwehr, wenn ein Notfall gemeldet wird.
This word order emphasizes schnell but retains correct German syntax.