Learn Hardcore German: Der Nachbar behauptet, dass der Lärm nicht von seiner Musik kommt. - The neighbor claims that the noise does not come from his music.

Der Nachbar behauptet, = The main clause (The neighbor claims)
dass der Lärm nicht von seiner Musik kommt. = The subordinate clause (that the noise does not come from his music)

In German, a comma is required before most subordinating conjunctions like dass, weil, obwohl, wenn, etc. So the comma is mandatory here, not optional.

Why is the verb kommt at the end of the dass-clause?

In German, subordinating conjunctions like dass send the conjugated verb to the end of the clause.

Main clause: Der Nachbar behauptet.
Verb behauptet is in the normal second position.
Subordinate clause: dass der Lärm nicht von seiner Musik kommt.
Subject: der Lärm
Negation + prepositional phrase: nicht von seiner Musik
Finite verb: kommt at the end

This is a standard rule: in dass-clauses, the conjugated verb comes at the end of the clause.

Why is it dass and not das?

dass (with double s) is a conjunction meaning that and introduces a subordinate clause.

das (one s) can be:

the neuter article the (das Haus)
a demonstrative that (das da = that there)
or a relative pronoun (das Buch, das ich lese = the book that I am reading)

In this sentence we introduce a content clause (that the noise doesn’t come from his music), so we must use the conjunction dass with double s:

Der Nachbar behauptet, dass ... = The neighbor claims that ...

What case is der Lärm, and why?

der Lärm is in the nominative case, because it is the subject of the subordinate clause.

In dass der Lärm nicht von seiner Musik kommt:

kommt = verb
Ask: Wer oder was kommt (nicht von seiner Musik)?
Answer: der Lärm → subject → nominative.

So:

der Lärm = nominative singular (masculine) of Lärm (noise).

Why is it von seiner Musik and not von seine Musik?

The preposition von always takes the dative case.

Noun: die Musik (feminine)
Dative singular feminine: der Musik
Possessive sein (his) in dative feminine becomes seiner.

So:

von + seiner Musik
= from his music
(literally: from the his-music in dative feminine)

seine Musik would be nominative or accusative feminine, which does not fit here because von requires the dative:

Correct: von seiner Musik
Incorrect: von seine Musik

What is the difference between behauptet and something simpler like sagt?

Both translate to says in some contexts, but they have different nuances:

sagen = to say, neutral, just reporting speech.
- Der Nachbar sagt, dass ... = The neighbor says that ...
behaupten = to claim / to assert, often implying:
- the statement might be doubtful
- or the speaker distances themself from it

So:

Der Nachbar behauptet, dass ... suggests:
The neighbor is insisting / claiming this, and we are not necessarily agreeing with him.

Can dass be omitted here, like in spoken English?

Yes, in spoken German dass is often dropped after verbs like sagen, meinen, glauben, behaupten, especially in informal speech.

So you might hear:

Der Nachbar behauptet, der Lärm kommt nicht von seiner Musik.

This is grammatically acceptable, particularly in spoken or more relaxed written German. However, in careful or formal writing, keeping dass (Der Nachbar behauptet, dass ...) is preferred and clearly correct.

Is Nachbar always masculine? What about a female neighbor?

Nachbar is the masculine noun meaning (male) neighbor.

Masculine singular: der Nachbar (the male neighbor)
Feminine singular: die Nachbarin (the female neighbor)
Plural (mixed or all male): die Nachbarn
Plural (all female, colloquially often just Nachbarn, sometimes Nachbarinnen): die Nachbarinnen

So Der Nachbar behauptet ... refers specifically to a male neighbor.
If the neighbor were female, you would say:

Die Nachbarin behauptet, dass der Lärm nicht von ihrer Musik kommt.

Why is nicht before von seiner Musik and not at the very end?

In German, nicht normally comes before the part of the sentence that is being negated.

Here:

von seiner Musik is the phrase being negated: the noise is not from his music.
So nicht directly precedes it:
dass der Lärm nicht von seiner Musik kommt.

You could also say:

dass der Lärm von seiner Musik nicht kommt.

Both are grammatically correct. The default and most neutral is:

nicht von seiner Musik kommt

von seiner Musik nicht kommt can sound more emphatic or contrastive, like:

The noise does indeed come from somewhere, but specifically not from his music.

Why is Musik capitalized?

In German, all nouns are capitalized, regardless of position in the sentence.

Nachbar (neighbor) → noun → capitalized
Lärm (noise) → noun → capitalized
Musik (music) → noun → capitalized

Verbs (kommt, behauptet), adjectives, and most other words are not capitalized, unless they are at the beginning of a sentence or part of a proper name.

Why is the tense behauptet (present) used and not a past tense like hat behauptet?

German often uses the present tense to describe:

something happening right now
something that is generally true or repeated
or something that is currently valid/ongoing

Der Nachbar behauptet, dass ... can mean:

The neighbor is currently claiming that,
or this is what he (generally) claims.

You could also say:

Der Nachbar hat behauptet, dass der Lärm nicht von seiner Musik kommt.
= The neighbor claimed (at some point in the past) that the noise does not come from his music.

Both are correct; the choice depends on the time frame you want to express. Here, the simple present is natural and common in German for reported speech that is still relevant now.