Mér er illt í hálsinum þegar ég tala mikið.

Because Icelandic often uses the dative for the person experiencing a feeling or bodily sensation.

So:

• ég = I (nominative)
• mér = to me / for me (dative)

In Mér er illt í hálsinum, the structure is literally closer to It is painful for me in the throat than to I hurt my throat.

If you said Ég er illur/ill, that would mean something more like I am bad, I am unwell, or in some contexts even evil, not my throat hurts.

A very literal breakdown is:

• Mér = to me / for me
• er = is
• illt = bad / painful
• í hálsinum = in the throat

So the whole thing is roughly:

To me, it is painful in the throat.

That sounds odd in English, but it is a very natural Icelandic way to talk about pain in a body part.

Illt is the neuter singular form of the adjective illur.

In Icelandic, certain impersonal expressions use the neuter singular form by default, especially when describing general states or sensations:

• Mér er kalt. = I am cold.
• Mér er heitt. = I am hot.
• Mér er illt. = I am in pain / It hurts.

So illt here is not agreeing with hálsinum. It is part of the fixed pattern mér er illt.

Because í can take different cases depending on meaning.

Here it means in in a location sense, so it takes the dative:

• hálsi = throat/neck (dative singular, indefinite)
• hálsinum = the throat/the neck (dative singular, definite)

So:

• í hálsi = in a throat / in a throat-area
• í hálsinum = in the throat

The ending -num shows that this is the definite dative singular form.

Because Icelandic often does not use a possessive with body parts when the owner is already clear.

Since mér already tells us who is experiencing the pain, í hálsinum naturally means in my throat in context.

This is very normal in Icelandic. English often says:

• my throat
• my head
• my back

But Icelandic often prefers:

• í hálsinum
• í höfðinu
• í bakinu

when the possessor is obvious.

It can refer to either the neck or the throat depending on context.

In this sentence, because of þegar ég tala mikið (when I talk a lot), the meaning is clearly throat rather than the outside of the neck.

So even though háls can be translated in different ways, here í hálsinum is best understood as in the throat.

Here þegar means when.

So:

• þegar ég tala mikið = when I talk a lot

It is a time clause, not a condition.

Compare:

• þegar = when
• ef = if

So this sentence means when I talk a lot, not if I talk a lot.

Here mikið means a lot, and it functions adverbially with the verb tala.

So:

• Ég tala mikið. = I talk a lot.

This is a common Icelandic pattern: the neuter singular form of an adjective can be used in an adverb-like way.

Compare:

• mikið = much / a lot
• lítið = little / not much

So tala mikið is simply the normal way to say talk a lot.

Yes. You can say:

Þegar ég tala mikið, er mér illt í hálsinum.

That is completely natural.

But notice the word order in the main clause:

• Mér er illt í hálsinum.
• Þegar ég tala mikið, er mér illt í hálsinum.

After a fronted clause, Icelandic usually puts the finite verb in second position in the main clause, so er comes before mér.

That word order often surprises English speakers.

Yes. Mér er vont í hálsinum is also natural and common.

Both illt and vont can be used for physical pain:

• Mér er illt í hálsinum.
• Mér er vont í hálsinum.

The difference is often small in everyday use, and which one sounds more natural can depend on speaker, region, and context. A learner should recognize both.

Yes, absolutely. This is a very useful Icelandic pattern.

The pattern is:

[dative person] + er + illt/vont + í + [body part in dative, often definite]

Examples:

• Mér er illt í höfðinu. = My head hurts.
• Henni er vont í bakinu. = Her back hurts.
• Honum er illt í fætinum. = His leg hurts.
• Þeim er vont í maganum. = Their stomach hurts.

So once you learn this structure, you can use it with many kinds of physical pain.

Yes, it is in the present tense:

• Mér er illt í hálsinum þegar ég tala mikið. = My throat hurts when I talk a lot.

In the past, you would usually change both verbs to past:

• Mér var illt í hálsinum þegar ég talaði mikið.

That means:

My throat hurt when I talked a lot
or
My throat used to hurt when I talked a lot, depending on context.

So the key changes are:

• er → var
• tala → talaði