Learn Hardcore Icelandic: Trefillinn er mjúkur og hlýr.

Trefillinn er mjúkur og hlýr.
= The scarf is soft and warm. (a specific scarf the speakers know about)
Trefill er mjúkur og hlýr.
= more like A scarf is soft and warm / A scarf (in general) is soft and warm.

In everyday speech, if you are talking about a particular scarf, you normally use the definite form trefillinn.

What is the grammatical gender of trefillinn, and how does that affect the adjectives?

Trefillinn is masculine.

In Icelandic, adjectives must agree with the noun in:

Gender (masculine / feminine / neuter)
Number (singular / plural)
Case (nominative / accusative / dative / genitive)

Here:

Noun: trefillinn = masculine, singular, nominative
Adjectives: mjúkur, hlýr
→ both are in masculine singular nominative to match trefillinn.

If the noun were neuter, for example, the adjective forms would change:

teppið er mjúkt og hlýtt
(The blanket is soft and warm. – teppið is neuter)

Why do the adjectives look like mjúkur and hlýr specifically? Could they have different endings?

Icelandic adjectives have different declension patterns: strong and weak forms.

In predicate position (after er, eru, etc.), you normally use the strong form:

Trefillinn er mjúkur og hlýr.
(The scarf is soft and warm.) – strong forms: mjúkur, hlýr

When the adjective is placed before a definite noun, it usually takes the weak form:

mjúkur trefill = a soft scarf (strong)
mjúki trefillinn = the soft scarf (weak adjective + definite noun)

So:

Strong masculine nominative singular: mjúkur, hlýr
Weak masculine nominative singular: mjúki, hlýi

In your sentence, the adjectives are strong because they come after the verb er.

What does er mean here, and how is the verb to be conjugated?

Er is the 3rd person singular present of vera = to be.

A mini present-tense paradigm:

ég er = I am
þú ert = you (sg.) are
hann / hún / það er = he / she / it is
við erum = we are
þið eruð = you (pl.) are
þeir / þær / þau eru = they are

So Trefillinn er mjúkur og hlýr = The scarf is soft and warm.

How do you pronounce Trefillinn er mjúkur og hlýr?

Approximate pronunciation (not strict IPA):

Trefillinn ≈ TREF-itl-in
- tr with an aspirated t
- e like in English bed
- ll has a little t sound, like tl
er ≈ air (short)
mjúkur ≈ MYU-kur
- mj like my
  - y sound
- ú like oo in food, but shorter and tenser
og often sounds like something between ok and og, frequently [ɔɣ]
hlýr ≈ something like hlyr, with:
- breathy hl cluster (the h makes the l voiceless)
- ý like a long ee sound
- final r pronounced clearly, but often quite light

Main stress in Icelandic is almost always on the first syllable of each word: TREF-illinn ER MJÚ-kur og HLÝR.

What do the double ll and nn in trefillinn tell me?

They matter for both spelling and pronunciation:

ll in Icelandic often represents a sound similar to tl (or dl) with a bit of a puff of air.
So trefill is not like English “tr-eh-fill”, but more like TREF-itl.
The -inn at the end marks the definite article for a masculine noun in the nominative:
trefill → trefillinn

So trefillinn is essentially trefill + inn, written together.

Is the word order fixed? Could I say Mjúkur og hlýr er trefillinn?

Normal, neutral Icelandic word order is:

Subject – Verb – Predicate
→ Trefillinn er mjúkur og hlýr.

You could encounter Mjúkur og hlýr er trefillinn in poetry or very stylized language, but in everyday speech and writing it would sound unusual or marked.

For learners, stick with:

[Subject] + er + [adjective(s)]
→ Trefillinn er mjúkur og hlýr.

Why is og used here, and can I change the order of the adjectives?

Og means and.

mjúkur og hlýr = soft and warm

You can switch the adjectives:

Trefillinn er hlýr og mjúkur.

The meaning is the same. The order is a matter of style and emphasis, not grammar.

How would I say The scarves are soft and warm?

You need the plural forms of the noun, verb, and adjectives.

Singular: Trefillinn er mjúkur og hlýr.
Plural: Treflarnir eru mjúkir og hlýir.

Changes:

trefillinn → treflarnir (the scarves)
er → eru (are)
mjúkur → mjúkir (masc. nom. plural)
hlýr → hlýir (masc. nom. plural)

What case is trefillinn in, and when would it change form?

In Trefillinn er mjúkur og hlýr, trefillinn is in the nominative case, because it is the subject of the sentence.

The noun changes form in other cases. For example:

Ég keypti trefilinn.
= I bought the scarf.
Here trefilinn is (definite) accusative as a direct object.
Ég er í treflinum.
= literally I am in the scarf (I’m wearing the scarf).
Here treflinum is dative after the preposition í.

The exact forms change according to case, but in your sentence we just see the nominative: trefillinn.

Is there a difference between hlýr and heitur for warm?

Yes, there is a nuance:

hlýr
- warm in a comforting, pleasant sense
- often used about clothes, rooms, a person’s personality, emotional warmth
- trefillinn er hlýr = the scarf keeps you comfortably warm
heitur
- hot / warm mainly in terms of temperature
- used for hot water, hot food, hot weather, etc.
- vatnið er heitt = the water is hot

So Trefillinn er hlýr sounds more like The scarf keeps you warm, which is exactly what you want for clothing.

How would I say The scarf is very soft and warm?

Add mjög (very) before the adjectives:

Trefillinn er mjög mjúkur og mjög hlýr.

You can also say it more compactly:

Trefillinn er mjög mjúkur og hlýr.

Here mjög clearly modifies mjúkur, and it’s normally understood that the whole description is strong, but if you want to be explicit for both, repeat mjög.