Learn Hardcore Icelandic: Bíllinn hans er nýr.

-inn is the suffixed definite article, meaning the noun is definite: bíll (car) → bíllinn (the car).
Here the car is the subject, so it’s in the nominative singular definite form.
Note the stem alternations in other cases with the article:
- Accusative: bílinn (I see the car: Ég sé bílinn.)
- Dative: bílnum (in the car: í bílnum)
- Genitive: bílsins (because of the car: vegna bílsins)

Why is the possessive hans after the noun, not before it?

Possessives like hans (his), hennar (her), þeirra (their), okkar (our), ykkar (your pl.) usually follow the noun: bíllinn hans.
Preposing hans (e.g., hans bíll) is archaic/poetic. In normal modern use, put it after the noun.
Possessives that behave like adjectives (minn/þinn/sinn) also most commonly follow the noun in everyday Icelandic: bíllinn minn, bíllinn þinn.

Why is it hans and not sinn?

sinn/sína/sitt is the reflexive possessive and refers back to the grammatical subject of the same clause.
In Bíllinn hans er nýr, the subject is the car (bíllinn), so using sinn would incorrectly mean “the car’s own car.”
Use sinn only when the subject is the possessor: Hann elskar bílinn sinn (He loves his own car).
In subordinate clauses, sinn refers to the subject of that clause, not the higher clause: Hann segir að bíllinn hans sé nýr (He says that his car is new). Using sinn there would be wrong.

Why is the adjective nýr (not nýi or nýjan)?

After the verb er (is), adjectives are predicative and take nominative agreement with the subject. Bíllinn is masculine nominative singular, so you use the strong form nýr.
Attributive adjectives before a definite noun take the weak form: nýi bíllinn (the new car).
Other forms you might see:
- nýr bíll (a new car; strong, attributive, indefinite)
- nýjan bíl (a new car; masculine accusative, e.g., Hann á nýjan bíl — He has a new car)

Could I say “the new car is his” instead? How?

Yes: Nýi bíllinn er hans.
- nýi is weak because the adjective is attributive to a definite noun (bíllinn).
- Compare: Bíllinn hans er nýr (His car is new) vs Nýi bíllinn er hans (The new car is his).

How would the sentence change if the owner were female or plural?

Female owner: Bíllinn hennar er nýr.
Plural owners: Bíllinn þeirra er nýr.
First/second person:
- Bíllinn minn/þinn/okkar/ykkar er nýr.
Note: The form of the possessive doesn’t affect the adjective; nýr agrees with the noun bíllinn (masculine), not with the owner.

How do I turn it into a yes/no question?

Invert by putting the verb first: Er bíllinn hans nýr?
Possible answers:
- Já, bíllinn hans er nýr.
- Nei, bíllinn hans er ekki nýr.

What’s the difference between Bíllinn hans and Bíll hans?

Bíllinn hans = the specific, known car of his (definite).
Bíll hans = a car of his (indefinite), often implying he has at least one car and you’re referring to one of them, not necessarily a specific one previously known in the context.

Which cases are used in the sentence?

Bíllinn: nominative singular definite (subject).
hans: genitive singular of the pronoun hann (he), used as a possessive.
er: 3rd person singular present of vera (to be).
nýr: nominative masculine singular adjective agreeing with bíllinn (predicate adjective).

How would forms change in other contexts with cases?

Accusative (direct object): Ég sé bílinn hans. (I see his car.)
Dative (after many preps): Ég sit í bílnum hans. (I sit in his car.)
Genitive (after certain preps): Vegna bílsins hans komum við seint. (Because of his car, we arrived late.)

Does the adjective change with different noun genders?

Yes, it agrees with the noun:

Any quick pronunciation tips for these words?

bíllinn: í/ý sound like a long “ee”; the ll is typically pronounced like a voiceless “tl” sound; so you’ll hear something close to “BEEL-tlin.”
hans: clear initial h; short “a” like “ah”; final s is voiceless.
er: short “ehr.”
nýr: long “ee” vowel; final trilled or tapped r.