Learn Hardcore Icelandic: Mér er kalt um hálsinn í dag.

The experiencer (the person who feels something) is in the dative case (mér, not ég).
The verb er and the adjective kalt do not agree with the person; they behave as if the sentence were impersonal: Það er kalt (It is cold).

This pattern is used for many feelings:

Mér er heitt. – I am hot.
Mér er kalt. – I am cold.
Mér er illt. – I am in pain / something hurts.

What case is mér, and why is that case used here?

Mér is the dative singular of ég (I).

This is a very common pattern: when you express how someone feels something physically, that person is often in the dative:

Mér er kalt. – I am cold.
Honum er kalt. – He is cold. (him-DAT is cold)
Henni er heitt. – She is hot. (her-DAT is hot)

You can think of it as “To me it is cold” rather than “I am cold”; that helps justify why Icelandic uses the dative.

Why is kalt in the neuter form and not kaldur / köld?

Kalt is the neuter singular form of the adjective kaldur (cold).

In the structure Mér er kalt, the adjective behaves as if it were describing an unspoken, neuter “it” (like Það er kalt – It is cold). Because of this:

The adjective is neuter singular: kalt.
It does not agree with mér (which is dative and has no gender by itself).

You only get forms like kaldur, köld, kaldrir, etc. when the adjective is directly modifying a noun or a personal subject in a normal way:

Kalt veður. – cold weather (neuter).
Kaldur dagur. – a cold day (masculine).
(But not Ég er kaldur for “I am cold” in the normal, everyday sense; you still say Mér er kalt.)

What does um mean in um hálsinn, and why is it used?

In isolation, um typically means around, about. Here, in um hálsinn, it is best understood as “around the neck” or “in the neck area”.

With body parts, um is often used with adjectives like kalt, heitt, etc., to specify where on the body you feel something:

Mér er kalt um hendurnar. – My hands are cold (I feel cold in/around my hands).
Mér er heitt um fætur. – My feet are hot/warm.

So Mér er kalt um hálsinn is literally “It is cold to me around the neck”, i.e. “My neck feels cold”.

Why is it hálsinn and not háls or hálsinum?

Hálsinn is the definite accusative singular form of háls (neck):

Nominative: háls – a neck
Accusative definite: hálsinn – the neck

Two important points:

The preposition um takes the accusative case, so we need the accusative form.
Icelandic usually uses a definite article with body parts instead of a possessive pronoun:
- um hálsinn – literally “around the neck”, but understood as “around my neck” in this sentence.

So you don’t say um minn háls (“around my neck”) in normal speech here; um hálsinn already carries that meaning in context.

Where is the word “my” in this sentence?

There is no separate word for “my”, but it is implied.

Icelandic very often uses definite body part + no possessive when the possessor is obvious:

Ég þvæi mér um hendur. – I wash my hands. (literally: I wash myself about the hands)
Mér er kalt um hálsinn. – My neck is cold. (literally: It is cold to me around the neck)

The combination of:

the dative experiencer (mér – to me) and
the definite form of the body part (hálsinn – the neck)

tells you that we mean “my neck”.

Can I change the word order, like Í dag er mér kalt um hálsinn?

Yes. All of the following are grammatical, with only slight differences in emphasis:

Mér er kalt um hálsinn í dag. – neutral; “I have a cold neck today.”
Í dag er mér kalt um hálsinn. – emphasises “today”.
Mér er í dag kalt um hálsinn. – also possible, but sounds a bit heavier / more formal.

Icelandic allows relatively flexible word order, especially with adverbials like í dag. The important core is:

[Dative experiencer] + er + kalt + um + [body part in acc. definite].

Is there a difference between Mér er kalt and Mér er kalt um hálsinn?

Yes:

Mér er kalt. – I am cold (in general).
Mér er kalt um hálsinn. – I feel cold specifically around my neck.

The phrase um hálsinn narrows the feeling down to a body part, just like in English when you say “My feet are cold” vs. “I am cold.”

Could I say Mér er kalt á hálsinum instead of um hálsinn?

You can say Mér er kalt á hálsinum, and it will be understood, but the nuance and pattern are slightly different:

um hálsinn (accusative) – standard phrasing with kalt / heitt for body parts; feels more idiomatic when talking about the area around something.
á hálsinum (dative) – literally “on the neck”; can sound a bit more like something is on the surface of your neck (for some speakers) or may simply be a less typical collocation with kalt.

Native usage tends to prefer um + acc. with these “feeling cold/hot in a body part” expressions:

Mér er kalt um hendur / um fætur / um hálsinn.

How would this sentence change if the person were he or she instead of I?

You only change the dative pronoun; the rest stays the same:

Mér er kalt um hálsinn í dag. – I am cold around the neck today.
Honum er kalt um hálsinn í dag. – He is cold around the neck today.
Henni er kalt um hálsinn í dag. – She is cold around the neck today.

Notice that:

kalt never changes,
hálsinn stays the same,
only mér / honum / henni changes.

How would I say “I am hot around the neck today”?

You can use the same structure but change kalt to heitt (neuter of heitur – hot):

Mér er heitt um hálsinn í dag.

Pattern:

Mér er heitt. – I am hot.
Mér er heitt um hálsinn. – My neck is hot / I feel hot around the neck.

How do you pronounce Mér er kalt um hálsinn roughly in English terms?

Very roughly (ignoring subtleties), you might think of it like:

Mér – like “myer” (some dialects closer to “mer” with a soft y sound).
er – like “air”.
kalt – like “kalt” in German; kal-t, short a as in “cup” but a bit back.
um – like short “oom”, but very short.
hálsinn – roughly HOWL-sin:
- há – like “how”
- ls – like in “pulse”
- inn – like “in”

So very roughly: MYER air kalt oom HOWL-sin (but the real Icelandic sounds are more compact and less English-like).