Learn Hardcore German: Später stellt der Nachbar die Musik leiser und sagt „Entschuldigung“. - Later the male neighbor turns the music down and says “Sorry.”

German main clauses follow the “verb-second” (V2) rule. Whatever you put in first position (here: the time adverb Später) is followed by the finite verb (stellt), and then the subject (der Nachbar).

Example variants: Später stellt der Nachbar … / Der Nachbar stellt später …
In a subordinate clause, the verb goes to the end: …, dass der Nachbar die Musik leiser stellt.

Why is the present tense used even though Später means “later”?

German often uses the present tense with a time adverb to talk about the future or to narrate events: Später stellt … can mean “Later, (he) will turn down…”. You could also use Futur I (wird … leiser stellen), but it’s not necessary. In storytelling, the “historical present” is also common.

What does leiser do here? Why not just leise?

Leiser is the comparative of leise (“quiet”), so it means “quieter.” With verbs like machen/stellen, German normally uses the comparative to express turning a setting down or up:

leiser = quieter, turning it down
lauter = louder, turning it up Using plain leise would mean “(make it) quiet/silent,” which is a different idea.

Why stellen? Could I say machen or drehen instead?

… die Musik leiser stellen is very idiomatic for adjusting a device/setting.
… die Musik leiser machen is also very common and neutral.
… leiser drehen / runterdrehen is colloquial, evoking the image of turning a knob.
More technical/formal: … die Lautstärke verringern. All work; stellen and machen are the safest neutral choices.

Is this a single verb like leiserstellen, or two words?

It’s two words: leiser stellen. The verb is stellen, and leiser is a comparative adjective telling you how it’s being set. Same with lauter stellen.

Why doesn’t leiser take an ending here?

Because leiser is used predicatively (it describes the resulting state of the object), not attributively before a noun. Attributive adjectives get endings (e.g., die leise Musik), but predicatives do not:

Attributive: die leise Musik
Predicative/result: die Musik leiser stellen

What cases are der Nachbar and die Musik?

der Nachbar is nominative (subject).
die Musik is accusative (direct object). Note: Nachbar is an N-declension noun: Genitive singular is des Nachbarn, dative dem Nachbarn, accusative den Nachbarn; plural die Nachbarn.

Should there be a colon before the quoted word? And where does the period go?

Preferred in formal writing: use a colon before direct speech and put the sentence-ending punctuation inside the quotes:

… und sagt: „Entschuldigung.“ Your version without a colon (… und sagt „Entschuldigung“.) is seen informally, but many style guides recommend the colon. If the interjection is exclaimed, use „Entschuldigung!“

Why is Entschuldigung capitalized, and why is there no article?

It’s a noun, so it’s capitalized. Here it’s used as a set interjection meaning “Sorry/Excuse me,” so no article is needed:

Interjection: Entschuldigung!
Literal noun “an apology”: eine Entschuldigung schreiben/bringen

Is sagt „Entschuldigung“ the same as entschuldigt sich?

Similar idea, different focus:

Er sagt „Entschuldigung.“ = He says “Sorry.”
Er entschuldigt sich (bei mir) (für …). = He apologizes (to me) (for …). The reflexive verb highlights the act of apologizing; the quoted form highlights the words spoken. Another common expression: Er bittet um Entschuldigung.

Can I put Später somewhere else in the sentence?

Yes. Common options:

Später stellt der Nachbar die Musik leiser … (time in first position)
Der Nachbar stellt später die Musik leiser …
Der Nachbar stellt die Musik später leiser … (also possible; it emphasizes that the turning-down happens later)

Why is there no comma before und?

In German, you don’t put a comma before coordinating und when it links two main-clause parts. Your sentence has one subject shared by two coordinated predicates: stellt … und sagt … No comma is needed.

Where did the subject go in … und sagt „Entschuldigung“?

It’s understood to be the same subject (der Nachbar) as in the first part. German often omits the repeated subject after und when it’s clear:

Full: …, und der Nachbar sagt …
Natural/elliptical: …, und sagt …

What’s the female form of der Nachbar?

die Nachbarin (plural: die Nachbarinnen). Gender-neutral phrasing in formal contexts can be something like Nachbarinnen und Nachbarn or typographic solutions such as Nachbar:innen (style-dependent).

How would I say “turn it up/down a bit/a lot”?

ein bisschen leiser/lauter stellen
deutlich/merklich leiser/lauter stellen
Colloquial: ein wenig runterdrehen / voll aufdrehen (turn down a bit / crank it up)