Learn Hardcore Icelandic: Ég vil sofa undir teppi. - I want to sleep under a blanket.

teppi on its own can mean “blanket” or “a blanket” depending on context.
If you want “the blanket”, you usually add a definite ending: teppið (nominative/accusative), teppinu (dative).

So:

Ég vil sofa undir teppi. = I want to sleep under a blanket / (some) blanket.
Ég vil sofa undir teppinu. = I want to sleep under the blanket.

Why is it sofa and not að sofa (“to sleep”) after vil?

In Icelandic, a group of verbs function somewhat like modal verbs in English (want, can, must, etc.). Common ones include:

vilja – to want
geta – to be able to / can
eiga – to be supposed to / ought to
skulu, mega, munu, ætla, etc.

When these verbs are followed by another verb, that second verb is used in the bare infinitive, without að:

Ég vil sofa. = I want to sleep.
Ég get sofað hér. = I can sleep here.
Ég á að sofa. = I am supposed to sleep.

So vil + sofa is the correct structure.
If vil were not there, you would normally use að: Ég ætla að sofa. = I’m going to sleep.

What is the difference between Ég vil sofa and Mig langar að sofa?

Both can translate as “I want to sleep”, but they feel different:

Ég vil sofa.
- Uses vilja (“to want”).
- Grammatically ordinary: Ég is the subject in nominative.
- Feels a bit more direct, sometimes more willful / determined:
  - I want to sleep (that’s my decision).
Mig langar að sofa.
- Uses langar, an impersonal verb meaning roughly “to long for / to feel like”.
- The person who wants something is in the accusative, not nominative:
  - Mig (accusative of ég) + langar
    - að sofa.
- Nuance is more like “I feel like sleeping / I’m in the mood to sleep.”

Both are correct; Ég vil sofa undir teppi is a straightforward, neutral way to say it.

What case is teppi in, and why? Doesn’t undir sometimes use a different case?

Yes, undir is one of the “two-way” prepositions in Icelandic. It can take:

Dative when describing location (where something is), and
Accusative when describing motion towards something (where something is moving to).

In Ég vil sofa undir teppi, the meaning is location: sleep (while being) under a blanket.
So undir takes the dative case here.

For the noun teppi (neuter), the singular forms are:

Nominative: teppi
Accusative: teppi
Dative: teppi
Genitive: teppis

So even though the case here is dative, the form just happens to look the same as nominative and accusative. That’s why you don’t see a visible change.

How would the sentence change if I wanted to say “under the blanket” instead of “under a blanket”?

You make the noun definite and keep the same case (dative, because of undir with location):

undir teppi = under a blanket (indefinite)
undir teppinu = under the blanket (definite, dative singular)

So your sentence would be:

Ég vil sofa undir teppinu. = I want to sleep under the blanket.

How do you conjugate the verb vilja (“to want”) in the present tense?

Present tense of vilja:

ég vil – I want
þú vilt – you (sg.) want
hann / hún / það vill – he / she / it wants
við viljum – we want
þið viljið – you (pl.) want
þeir / þær / þau vilja – they want

So Ég vil sofa undir teppi is using ég vil, the 1st person singular present form.

What about sofa – how does that verb behave in other tenses?

sofa is a strong verb with vowel changes. Key forms:

Infinitive: sofa – to sleep
Present:
- ég sef – I sleep
- þú sefur – you sleep
- hann/hún/það sefur – he/she/it sleeps
- við sofum – we sleep
- þið sofið – you (pl.) sleep
- þeir/þær/þau sofa – they sleep
Past (simple):
- ég svaf – I slept
- við sváfum – we slept
Past participle / supine: sofið

Examples:

Ég sef undir teppi. – I sleep under a blanket.
Ég svaf undir teppi. – I slept under a blanket.
Ég hef sofið undir teppi. – I have slept under a blanket.

Can I change the word order, like Ég vil undir teppi sofa?

In this sentence, the natural word order is:

Ég vil sofa undir teppi.

Icelandic is more flexible than English, but this particular kind of splitting (Ég vil undir teppi sofa) is not natural here and would sound strange.

You can move undir teppi as a whole if you want to emphasize it, usually with some context or intonation:

Ég vil undir teppi sofa – possible in very marked, poetic, or strongly emphasized speech, but not something you should copy as a learner.

For normal, neutral speech, keep:

subject – modal verb – main verb – prepositional phrase
Ég vil sofa undir teppi.

Is teppi singular or plural here? How would I say “blankets”?

In Ég vil sofa undir teppi, teppi is singular (one blanket).

The noun teppi (neuter) has:

Singular:
- Nom/Acc: teppi – blanket
- Dat: teppi
- Gen: teppis
Plural:
- Nom/Acc: teppi – blankets
- Dat: teppum
- Gen: teppa

So the plural nominative/accusative is also teppi. That means context must tell you whether it’s singular or plural.

To clearly mean “under blankets” (plural), you would normally make that clear with number or context:

Ég vil sofa undir mörgum teppum. – I want to sleep under many blankets.
Ég vil sofa undir teppum. – I want to sleep under blankets. (dative plural, more clearly plural than just undir teppi)

How is Ég vil sofa undir teppi pronounced?

Approximate pronunciation (in simple English-like terms):

Ég – like “yeh” or “yei”; often [jɛː] or [jei̯]
vil – “vil” with a short i (like fill), [vɪl]
sofa – roughly “SOH-va”, [sɔːva]
undir – roughly “UHN-dir”; u like in put, [ʏntɪr]
teppi – roughly “TEH-pi”, with:
- t strongly aspirated (a little puff of air): [tʰ]
- pp pronounced as a long p, which makes the e short: [tʰɛhpɪ]

Said smoothly: [jɛː vɪl sɔːva ʏntɪr tʰɛhpɪ] (details vary slightly by dialect and speaker).