Learn Hardcore German: Der Verkehr ist laut.

In German every noun has a gender (masculine, feminine, neuter). Verkehr happens to be masculine (der). Unfortunately there’s no rule that tells you exactly why; you have to learn each noun’s gender as you go or use memorization tricks (e.g., picturing traffic signs with the color blue for “der”).

What case is Der Verkehr in, and how do I know that?

Der Verkehr is in the nominative case because it’s the subject of the sentence (it’s performing the action of the verb ist). In German, the subject of a finite verb is always in the nominative.

Why doesn’t laut get an ending here (like lauter or lautes)?

When an adjective follows a form of sein (to be) and describes the subject, it’s used predicatively. Predicative adjectives in German remain uninflected—they don’t take endings. Examples:

Das Haus ist groß.
Der Film ist spannend.

Can I use laut as an adverb, too?

Yes. laut can also function as an adverb meaning “out loud” or “in a loud voice.” In adverbial use it stays unchanged:

Er spricht laut. (“He speaks loudly / out loud.”)
Sie lacht laut. (“She laughs loudly.”)

What’s the difference between laut and lärmig?

laut simply means “loud” (high volume). You can say Der Verkehr ist laut, focusing on how high the noise level is.
lärmig means “noisy” in the sense of causing disturbance or nuisance (lots of noise or racket). You might describe a party or machines as lärmig because they’re bothersome.

Can I say Der Verkehr ist sehr laut?

Absolutely. You can intensify the adjective with an adverb like sehr:

Der Verkehr ist sehr laut. (“The traffic is very loud.”)

Is Verkehr countable or uncountable in German?

Verkehr is generally uncountable (a mass noun) in German, just like in English. You don’t use a plural form Verkehre in ordinary contexts; you talk about der Verkehr as a general concept.

Why do we need Der in this sentence? Can we drop it?

In standard German you normally need the definite article before a singular count or mass noun in a main clause. If you drop der, you get Verkehr ist laut, which sounds like a headline or a very clipped statement—it’s grammatically possible but not typical in spoken or written full sentences.