Learn Hardcore Icelandic: Teppið á rúminu er þykkt og hlýtt. - The blanket on the bed is thick and warm.

In Icelandic, “the” is usually not a separate word as in English. Instead, it is attached to the end of the noun as a suffix, and its exact form changes depending on gender, number, and case.

So:

English: the blanket
Icelandic: teppið (one word, with -ið carrying the meaning of “the”)

Why is it rúminu and not just rúm or rúmið?

Rúminu is dative singular, definite of rúm (bed).

Base forms:

rúm = bed (indefinite)
rúmið = the bed (nominative/accusative singular, definite)
rúmi = bed (dative singular, indefinite)
rúminu = the bed (dative singular, definite)

In the phrase á rúminu (on the bed), the preposition á requires the dative case when you describe location (where something is). So:

á rúminu = on the bed (location → dative)
á rúmið would be used if there is movement onto the bed (direction → accusative), e.g. Ég set teppið á rúmið = I put the blanket on the bed.

What exactly is the role of the preposition á here? Does it always mean “on”?

In this sentence, á means “on” in the sense of location:

Teppið á rúminu er þykkt og hlýtt.
The blanket on the bed is thick and warm.

The preposition á in Icelandic is flexible; it can mean:

on, onto
in, into (in some expressions)
sometimes even at

Its meaning is also tied to the case it governs:

á
- dative → usually location (where?)
  - Teppið er á rúminu. = The blanket is on the bed.
á
- accusative → usually movement/direction (where to?)
  - Ég set teppið á rúmið. = I put the blanket onto the bed.

In your sentence, it’s location, so á rúminu (dative).

Why do þykkt and hlýtt both end in -tt?

Both þykkt (thick) and hlýtt (warm) are neuter, singular, nominative forms of their adjectives, agreeing with teppið, which is a neuter singular noun in the nominative.

Dictionary (base) forms of the adjectives:

þykkur = thick (masculine nominative singular)
hlýr = warm (masculine nominative singular)

Neuter forms (strong declension, nominative singular):

þykkt = thick (neuter)
hlýtt = warm (neuter)

Because Teppið is:

neuter
singular
subject (nominative case),

the adjectives that describe it must match these features, so we get:

Teppið er þykkt og hlýtt.

That matching of gender, number, and case between nouns and adjectives is required in Icelandic.

What are the dictionary forms of þykkt and hlýtt, and how do they change with gender?

The dictionary (masculine nominative singular) forms are:

þykkur = thick
hlýr = warm

Nominative singular forms by gender:

þykkur (thick)

masculine: þykkur stóll (a thick chair – e.g. padded)
feminine: þykk bók (a thick book)
neuter: þykkt teppi (a thick blanket)

hlýr (warm)

masculine: hlýr jakki (a warm jacket)
feminine: hlý peysa (a warm sweater)
neuter: hlýtt teppi (a warm blanket)

In your sentence, teppi is neuter, so the adjectives must be neuter nominative singular: þykkt, hlýtt.

Why do we say Teppið er þykkt og hlýtt, but þykka teppið when the adjective is before the noun?

Icelandic adjectives have two main patterns: strong and weak declension.

After “to be” (er), in predicate position, the adjective usually takes the strong form, even if the noun is definite:
- Teppið er þykkt. = The blanket is thick.
- Húsið er nýtt. = The house is new.
Before a definite noun, the adjective usually takes the weak form:
- þykka teppið = the thick blanket (weak)
- nýja húsið = the new house (weak)

So:

Teppið er þykkt. → strong form þykkt (predicate)
þykka teppið → weak form þykka (attributive, in front of a definite noun)

Your sentence uses the predicate pattern: Teppið … er þykkt og hlýtt.

Can I switch the order and say Teppið er hlýtt og þykkt instead?

Yes, you can. Both are grammatically correct:

Teppið er þykkt og hlýtt.
Teppið er hlýtt og þykkt.

In everyday speech, the difference is only a matter of emphasis or style; whichever adjective comes first might feel slightly more prominent, but practically they are interchangeable here.

Can I change the word order to Teppið er þykkt og hlýtt á rúminu?

You can say Teppið er þykkt og hlýtt á rúminu, but the meaning shifts slightly.

Teppið á rúminu er þykkt og hlýtt.
Focus is on which blanket: specifically the blanket on the bed (as opposed to some other blanket somewhere else).
Teppið er þykkt og hlýtt á rúminu.
Sounds more like: The blanket is thick and warm *when it’s on the bed, or *on the bed, the blanket is thick and warm.
The phrase á rúminu now feels more like it’s qualifying the situation, not just identifying which blanket.

For the simple identifying meaning “The blanket on the bed is thick and warm”, the original order Teppið á rúminu er þykkt og hlýtt is the most natural.

How should I pronounce teppið and hlýtt? The double consonants and the hl- look tricky.

Key points:

Teppið

te: like “teh” (short e, not like English “tee”)
pp: a strong, unaspirated p, with a short vowel before it. Think of a very short vowel plus a tight p.
-ið: roughly “ith” with a very soft ð (like a soft, voiced th in “this”), sometimes almost just [ɪ] for learners.

Rough guide: TEH-pith (short, clipped p and i).

hlýtt

hl: pronounce h and l together; the h is audible, unlike English “l” by itself.
ý: long vowel, like a long ee, but with the tongue slightly further forward and lips more spread.
tt: again, a strong, unaspirated t, with the vowel cut short.

Approximate English-friendly version: HLEE-tt (with a long ee and clear h).

Stress in Icelandic almost always falls on the first syllable, so:

TÉP-pið
HLÝTT (single-syllable, so stress is obvious)

What is the difference between teppi, sæng, and rúmteppi? Are they all “blanket”?

They’re all bedding-related, but not identical:

teppi
General word for a blanket, a covering you put over yourself. Can be used on a bed, sofa, etc.
sæng
Usually refers to a duvet / comforter (the thick, fluffy thing you sleep under, often in a cover).
rúmteppi
Literally “bed-blanket” or “bedspread” – more like a decorative or protective cover that lies on top of the bed rather than the blanket you sleep under.

In your sentence, Teppið á rúminu suggests the blanket on the bed, which in context is more like the blanket you sleep under, so teppi is appropriate.