Learn Hardcore Icelandic: Bak mitt er hlýtt undir teppinu.

In the sentence Bak mitt er hlýtt undir teppinu., why is the possessive mitt placed after bak instead of before, like in English my back?

In Icelandic, the normal position for possessive pronouns (my, your, his, etc.) is after the noun:

bók mín – my book
bíllinn minn – my car
bak mitt – my back

Putting the possessive before the noun is also possible, but it tends to sound more emphatic or stylistically marked, a bit like stressing it in English:

mitt bak – my back (as opposed to someone else’s, or with some emphasis)

So bak mitt is the neutral, everyday way to say my back in this kind of sentence.

Why is the form mitt used and not minn, mín, or mínum?

The form of the possessive pronoun has to agree with the gender, number, and case of the noun it modifies.

bak is a neuter noun in Icelandic.
Here it is the subject of the sentence, so it is in the nominative singular.

The possessive “my” declines like this (singular forms):

Masculine: minn (nom), mínn (acc), mínum (dat), míns (gen)
Feminine: mín, mína, minni, minnar
Neuter: mitt (nom/acc), mínu (dat), míns (gen)

Since bak is neuter nominative singular, you must choose the neuter nominative/accusative form:

bak mitt – my back

Forms like minn (masc), mín (fem), or mínum (dative) would not agree grammatically with bak, so they would be incorrect here.

Why is the adjective hlýtt and not hlýr or hlý?

Adjectives in Icelandic must agree with the noun they describe in:

gender (masc/fem/neut)
number (sing/pl)
case (nom/acc/dat/gen)

Here:

The noun is bak (neuter, singular, nominative).
The sentence is Bak mitt er hlýtt … – so hlýtt is a predicate adjective describing bak.

The basic (masculine nominative singular) form of the adjective is hlýr (warm). Its neuter nominative singular form is hlýtt. So you get:

Masculine: hlýr
Feminine: hlý
Neuter: hlýtt

Because bak is neuter, the adjective has to be neuter too:

Bak mitt er hlýtt – My back is warm.

Hlýr or hlý would not agree with a neuter noun.

What is the difference between Bak mitt er hlýtt … and something like Bakinu mínu er hlýtt …?

Both patterns are possible, but they have slightly different structures and feel:

Bak mitt er hlýtt undir teppinu.
- Literally: My back is warm under the blanket.
- Bak mitt is a straightforward nominative subject.
- This sounds like a simple statement that your back has the property of being warm (like The room is warm).
Bakinu mínu er hlýtt undir teppinu.
- Literally: To my back is warm under the blanket.
- Bakinu mínu is in dative; this is an “impersonal” construction, similar to Mér er kalt (I am cold / It is cold to me).
- This version emphasizes more the experience/feeling of warmth on your back, a bit like saying My back feels warm.

Both can be understood, but the original sentence with nominative Bak mitt is structurally simpler for a learner and is a very natural sentence.

Why is it undir teppinu and not undir teppið?

The preposition undir (under) is one of the so‑called two-way prepositions in Icelandic. It can take:

Dative when talking about location (where something is)
Accusative when talking about motion (to where something moves)

In your sentence, we describe a static location (where the back is warm):

undir teppinu – under the blanket (no movement → dative)

If there were motion toward the space under the blanket, you would use accusative:

Hann fer undir teppið. – He goes under the blanket. (movement → accusative)

So here undir teppinu is correct because it describes where the back is warm, not movement into that place.

What exactly is happening morphologically in teppinu?

The base noun is teppi – blanket, a neuter noun. Its singular forms are roughly:

Nom/Acc: teppi – a blanket
Dat: teppi – to/under/with a blanket
Gen: teppis

When you add the definite article (the) as a suffix, it changes:

Nom/Acc definite: teppið – the blanket
Dat definite: teppinu – to/under/with the blanket
Gen definite: teppisins

In undir teppinu:

undir takes dative for location.
You want “the blanket”, so you use dative definite → teppinu.

So teppinu = teppi (blanket) + -nu (part of the definite dative ending in the neuter).

Why is there no separate word for “the” in teppinu? Why isn’t it something like undir the teppi?

Icelandic doesn’t use a separate word for “the” like English does. Instead, the definite article is usually attached to the end of the noun as a suffix:

teppi – blanket
teppið – the blanket (nom/acc)
teppinu – the blanket (dative)
bók – book
bókin – the book

So teppinu already contains the meaning of “the blanket”.
undir teppinu = under the blanket.

Could the word order be Bak mitt undir teppinu er hlýtt instead?

That word order is technically possible, but it would be unusual and sound marked in a neutral context.

Normal Icelandic main-clause word order when you simply state something is:

Subject – Verb – Complement – (extra information)

So you get:

Bak mitt er hlýtt undir teppinu.
Subject = Bak mitt
Verb = er
Complement (adjective) = hlýtt
Extra phrase = undir teppinu

Moving undir teppinu earlier, as in Bak mitt undir teppinu er hlýtt, draws attention to that phrase in a way that sounds somewhat poetic, stylistic, or just odd in everyday speech. For learners, it is best to stick with:

Bak mitt er hlýtt undir teppinu.

In English we often say “My back feels warm” instead of “My back is warm.” Why does Icelandic say er hlýtt (is warm) rather than a verb like “feel”?

Icelandic very often uses the plain copula vera (er = is) plus an adjective to express sensations that English might describe with “feel”:

Mér er kalt. – I am cold / I feel cold.
Mér er heitt. – I am hot / I feel hot.
Fæturnir mínir eru þreyttir. – My feet are tired / My feet feel tired.
Bak mitt er hlýtt. – My back is warm / My back feels warm.

So Bak mitt er hlýtt naturally covers the English meaning “My back feels warm” without needing a separate verb equivalent to “feel” here.

Is bak always the word for “back” (like the back of the body), or can it mean other things?

Bak in this sentence is indeed the back of the body. It’s the normal word for your back as a body part:

Ég er með verk í bakinu. – I have pain in my back.

There are related or similar-looking words:

bakki – tray, bank/shore (river bank), or shelf
bakgrunnur – background
bakvið – behind (preposition/adverb)

But for the body part, the standard word is bak, and that’s what you have in Bak mitt er hlýtt ….

Could I replace hlýtt with heitt here? What is the difference?

Yes, you could say:

Bak mitt er heitt undir teppinu.

The difference is basically in degree of warmth and nuance:

hlýtt – warm, pleasantly warm, cozy
heitt – hot, very warm (sometimes uncomfortably so)

So:

Bak mitt er hlýtt undir teppinu.
– My back is (nicely) warm under the blanket.
Bak mitt er heitt undir teppinu.
– My back is hot under the blanket (could sound like it’s too warm, or you’re overheating).

Context will decide which one sounds more natural. For comfortable warmth, hlýtt is usually better.

Is Bak mitt er hlýtt undir teppinu. a complete, natural sentence in Icelandic, or would a native speaker usually phrase it differently?

It is a complete and grammatical sentence, and a native speaker can absolutely say it.

In everyday speech, Icelandic also often uses slightly different patterns to talk about how parts of the body feel, for example:

Mér er hlýtt á bakinu. – I am warm on the back / My back is warm.
Bakinu mínu er hlýtt undir teppinu. – My back feels warm under the blanket.

But Bak mitt er hlýtt undir teppinu. is perfectly understandable and natural, and it’s a very good sentence for a learner to analyze because it shows:

postposed possessive (bak mitt)
adjective agreement (hlýtt with a neuter noun)
a two-way preposition with dative for location (undir teppinu).