Learn Hardcore Icelandic: Peysan hennar er græn og mjög hlý.

Icelandic usually does not use a separate word for the. Instead, it adds a definite ending to the noun itself. For feminine nouns ending in -a, the nominative singular definite ending is -n, which merges with the a and becomes -an:

peysa = a sweater
peysan = the sweater

So peysan is simply peysa + -n (definite article) → the sweater.

What exactly does hennar mean, and how is it different from hún?

Hennar means her / of her, expressing possession:

peysan hennar = her sweater (literally: the sweater of her)

Grammatically:

hún = she (nominative, subject form)
hennar = her (genitive, possessive form, “of her”)

You use hennar after a noun to show that something belongs to a female person. For example:

tölvan hennar = her computer
bíllinn hennar = her car

So in Peysan hennar er græn og mjög hlý, hennar shows that the sweater belongs to a woman or girl.

Why does hennar come after peysan instead of before it, like in English?

English says her sweater (possessive before the noun), but Icelandic normally puts 3rd‑person possessives after the noun:

peysan hennar = her sweater
bókin hans = his book
börnin þeirra = their children

You can say hennar peysa, but that usually has special emphasis, like stressing HER sweater (as opposed to someone else’s). The neutral, everyday order is:

[noun] + [possessor] → peysan hennar

Why are græn and hlý in the feminine form?

In Icelandic, adjectives must agree with the noun they describe in:

gender (masculine / feminine / neuter)
number (singular / plural)
case (nominative, accusative, etc.)

The subject here is peysan, which is:

feminine
singular
nominative (subject)

So the adjectives that describe it must also be feminine singular nominative. The strong forms are:

grænn (masc), græn (fem), grænt (neut)
hlýr (masc), hlý (fem), hlýtt (neut)

Because the noun is feminine, you get:

græn (fem. sg. nom.)
hlý (fem. sg. nom.)

Hence: Peysan hennar er græn og mjög hlý.

Why is it græn and not grænn or græna?

All three forms belong to the adjective whose base form is grænn (green), but they are used in different situations:

grænn = masculine nominative singular (strong)
græn = feminine nominative singular (strong)
græna = weak form used with definite nouns before the noun
- e.g. græna peysan = the green sweater

In your sentence, græn comes after the verb er (is). This is called a predicative adjective (an adjective used with to be). Predicative adjectives use the strong form, and they agree in gender, number, and case with the subject:

Peysan hennar er græn. = Her sweater is green.
- subject: peysan (fem. sg. nom.)
- adjective: græn (fem. sg. nom., strong)

If the adjective came before the noun in a definite phrase, you would normally see the weak form:

græna peysan = the green sweater (attributive, weak declension)

So græn is correct here because it is used after er, describing the subject.

Why is it hlý and not hlýr or hlýtt?

The base adjective is hlýr (warm), and its strong nominative singular forms are:

masculine: hlýr
feminine: hlý
neuter: hlýtt

Since peysan is feminine, the adjective must also be feminine nominative singular, so we choose:

hlý

If the noun were masculine or neuter, you would change the form:

Jakkinn er hlýr. = The jacket is warm. (masculine)
Teppið er hlýtt. = The blanket is warm. (neuter)

What does mjög mean, and where does it go in the sentence?

Mjög is an adverb meaning very. It usually goes directly before the adjective it modifies:

mjög hlý = very warm
mjög græn = very green

In your sentence:

Peysan hennar er græn og mjög hlý.
→ mjög is modifying hlý (only the warmth), so the meaning is Her sweater is green and very warm.

If you want very to modify a different adjective, you move mjög in front of that adjective:

Peysan hennar er mjög græn og hlý.
→ Her sweater is very green and warm.

And if you want very for both adjectives, the safest and clearest way is to repeat it:

Peysan hennar er mjög græn og mjög hlý.
→ Her sweater is very green and very warm.

Does mjög here apply to both græn and hlý, or just to hlý?

As the sentence is written:

Peysan hennar er græn og mjög hlý.

Mjög is placed right before hlý, so it naturally modifies hlý only. That gives:

græn = green
mjög hlý = very warm

So the most straightforward reading is:

Her sweater is green and very warm.

If you want mjög to clearly apply to both adjectives, repeat it:

Peysan hennar er mjög græn og mjög hlý.

What does er do in this sentence? Is it just the same as English is?

Yes. Er is the 3rd person singular present tense of the verb vera (to be), and it works like English is here:

Peysan hennar = her sweater (subject)
er = is
græn og mjög hlý = green and very warm (what the subject is)

So the structure is very close to English:

Peysan hennar er græn og mjög hlý.
= Her sweater is green and very warm.

What cases are used in this sentence?

The overall structure is:

Peysan hennar (subject noun phrase)
er (copula verb)
græn og mjög hlý (predicative adjectives)

Case by case:

peysan is nominative singular, because it is the subject.
hennar is genitive singular, because it marks possession (of her).
- The noun phrase peysan hennar as a whole is in nominative case, but inside it, the possessor hennar is genitive.
græn and hlý are nominative singular as well, agreeing with the subject in gender, number, and case after the verb er.

So you see a typical Icelandic pattern:

Subject noun: nominative
Possessor inside the subject: genitive
Adjectives after er: nominative, agreeing with the subject

How would the sentence change with a neuter or plural noun instead of peysa?

The adjectives would change to match the gender and number of the new noun.

Neuter singular example – teppi (blanket, neuter):
- teppi (a blanket) → teppið (the blanket)
- Feminine → neuter changes the adjective endings:
Teppið hennar er grænt og mjög hlýtt.
Her blanket is green and very warm.
- grænt = neuter nom. sg. (from grænn)
- hlýtt = neuter nom. sg. (from hlýr)
Feminine plural example – peysur (sweaters, plural):
- peysur (sweaters) → peysurnar (the sweaters)
Peysurnar hennar eru grænar og mjög hlýjar.
Her sweaters are green and very warm.
- eru = plural of er
- grænar, hlýjar = feminine nominative plural strong forms

How do you pronounce Peysan hennar er græn og mjög hlý?

Approximate pronunciation (stressed syllables in caps):

Peysan → PEY-san
- ey like the vowel in English say
hennar → HEN-nar
er → ehr (short, like air but quicker)
græn → something like GRINE, rhyming with fine (with a rolled r and Icelandic æ like English eye)
og → roughly ohg; the g may be very soft or almost silent in fast speech
mjög → myœg, where jö is like German ö in schön; you can approximate it as myurg or my-erg with a rounded vowel
hlý → HLEE, with a long ee sound; the hl is like l with a bit of breath

Said together, with main stress on the first syllable of Peysan and light stress on the first syllable of hennar:

PEY-san HEN-nar er GRÆN og MJÖG hlý.