Learn Hardcore Dutch: De buurjongen is luidruchtig, maar zijn zus is heel rustig. - The neighbor boy is noisy, but his sister is very calm.

All of these normally take the article de: de buurjongen, de buurman, de buurvrouw, het buurmeisje (because meisje is a diminutive and all diminutives take het).

Why is it de buurjongen and not het buurjongen?

In Dutch, every noun has a grammatical gender:

de-words: common gender
het-words: neuter gender

Jongen (boy) is a person and takes de: de jongen.
The compound buurjongen keeps that same gender, so it is also a de-word:

de jongen → de buurjongen

So:

de buurjongen = the neighbour boy
het buurmeisje = the neighbour girl (because meisje is neuter due to the -je).

Why is zijn used in maar zijn zus? Does it mean his or her?

In this sentence, zijn means his and refers back to de buurjongen:

de buurjongen (he)
zijn zus = his sister

So literally: The neighbour boy is noisy, but his sister is very calm.

A few notes:

zijn = his
haar = her

If the possessor is clearly male, you use zijn.
If the possessor is clearly female, you use haar:

De buurvrouw is luidruchtig, maar haar zus is heel rustig.
(The neighbour woman is noisy, but her sister is very calm.)

In more general or impersonal contexts, Dutch sometimes uses zijn as a default, but here it’s clearly matching the boy.

Why is there no extra ending on luidruchtig and rustig? Should it be luidruchtige or rustige?

Dutch adjectives only take the -e ending in certain positions. The key rule:

Before a noun → usually add -e
After a verb like zijn (to be) → no -e

In this sentence, both adjectives are used after the verb is:

De buurjongen is luidruchtig.
Zijn zus is heel rustig.

Here they describe the subject via the verb is, so they stay in the base form: luidruchtig, rustig.

Compare:

De luidruchtige buurjongen (the noisy neighbour boy) – before noun → luidruchtige
De rustige zus (the calm sister) – before noun → rustige

What is the role of heel in heel rustig? Is it like “very”?

Yes. Heel in heel rustig works as an intensifier and is close to very or really:

heel rustig = very calm / really calm

Other common intensifiers:

erg rustig = very calm
zeer rustig = very calm (a bit more formal)
ontzettend rustig = extremely calm

Here, heel is an adverb modifying the adjective rustig, so it comes directly before it:

Ze is rustig. → She is calm.
Ze is heel rustig. → She is very calm.

What exactly does luidruchtig mean? Is it the same as just luid?

Luidruchtig roughly means noisy in the sense of making a lot of noise / being loud and rowdy.

Nuances:

luid = loud (volume, like audio level)
- De muziek is luid. – The music is loud.
luidruchtig = noisy, boisterous
- De buurjongen is luidruchtig. – The neighbour boy is noisy (probably talks, shouts, plays loudly, etc.)

Other common words for a noisy/active person:

druk – busy, hyper, lively (often used for children)
herrie schoppend – literally: making a racket (colloquial)

So luidruchtig focuses more on the behavior causing noise than on just sound volume.

Why is there a comma before maar, and does maar change the word order?

Maar is a coordinating conjunction, like but in English. It connects two main clauses:

De buurjongen is luidruchtig,
maar zijn zus is heel rustig.

You normally put a comma before maar when it links two full clauses.

Because maar is a coordinating conjunction, it does not cause inversion (it doesn’t push the verb to the end like some other conjunctions do). Both clauses keep normal main-clause word order:

Subject – Verb – Rest of sentence
De buurjongen – is – luidruchtig
zijn zus – is – heel rustig

By contrast, subordinating conjunctions such as omdat, dat, als send the verb to (almost) the end:

Omdat zijn zus heel rustig is, is de buurjongen extra luidruchtig.

Why is the word order zijn zus is heel rustig and not something like is zijn zus heel rustig?

In a statement (not a question), Dutch main clauses usually follow this pattern:

Subject – Verb – (Other elements)

Here:

zijn zus = subject
is = verb
heel rustig = complement

So the normal order is:

Zijn zus is heel rustig.

You would move the verb in front of the subject only in a yes/no question or after some fronted elements:

Is zijn zus heel rustig? – Is his sister very calm?
Heel rustig is zijn zus niet. – Very calm his sister is not. (emphatic / stylistic)

How would you make this sentence plural, talking about several neighbour boys and their sisters?

First change the nouns to the plural and adjust possessives:

de buurjongen → de buurjongens (neighbour boys)
zijn zus (his sister) → usually becomes hun zussen (their sisters)

Possible plural version:

De buurjongens zijn luidruchtig, maar hun zussen zijn heel rustig.
→ The neighbour boys are noisy, but their sisters are very calm.

Notes:

hun = their
zus → zussen (sister → sisters)

How do you pronounce buurjongen, luidruchtig, and rustig?

Approximate English-based guide (not exact IPA, but helpful):

buurjongen ≈ BOOR-yong-en
- uu = long “u” like French u in tu or German ü*
- j = English y in yes
- ngen = like ng-en
luidruchtig ≈ LOID-rukh-tikh
- lui = roughly like loy in boy, but with rounded lips
- ch = a guttural sound like the ch in German Bach or Scottish loch (in both rucht and tig)
- tig here does not sound like English -tig in tiger; final g is also that guttural ch-type sound
rustig ≈ RUS-tikh
- u = like the French u in tu or somewhere between English i in bit and u in put
- g at the end = same guttural ch sound

If you know German: Dutch g and ch are similar to German ch in ach.