Learn Hardcore Icelandic: Munnurinn er þurr þegar ég tala mikið.

You can say Munnurinn minn er þurr for “My mouth is dry”, but the possessive minn is usually omitted when it’s obvious whose body part is meant.

What is the difference between munnur, munninn, and munnurinn?

All three come from the noun munnur (mouth), masculine:

munnur – indefinite, nominative singular: “a mouth”
munnurinn – definite, nominative singular: “the mouth” (used as subject here)
munninn – definite, accusative singular: e.g. í munninn = “into the mouth”

In the sentence Munnurinn er þurr, munnurinn is the subject, so it must be nominative definite.

Why is the adjective þurr in this exact form?

Adjectives in Icelandic agree with the noun in:

gender
number
case

Here:

munnurinn is masculine, singular, nominative
so þurr is masculine, singular, nominative too

If the noun changed, the adjective would change, e.g.:

maturinn er þurr – the food is dry (masc.)
súpan er þurr would be wrong; you’d say súpan er þykk etc. (different adjective, feminine agreement)

Why is the verb er used, and what form is it?

er is the 3rd person singular present of vera (to be):

ég er – I am
þú ert – you are
hann/hún/það er – he/she/it is

The subject munnurinn is 3rd person singular (it), so you must use er:

Munnurinn er þurr – The mouth is dry.

Is the word order Munnurinn er þurr þegar ég tala mikið fixed, or can it change?

You can also say:

Þegar ég tala mikið, er munnurinn þurr.

Both are correct.

In the original, the main clause comes first, then the subordinate clause with þegar.
In the second version, the subordinate clause is fronted.

The verb er still comes right after the first element of the main clause (munnurinn / the whole Þegar… clause), in line with Icelandic main‑clause word order rules.

What exactly does þegar do in this sentence?

þegar is a conjunction meaning “when”:

þegar ég tala mikið = when I talk a lot

It introduces a time clause (a subordinate clause).
Inside that clause, the word order is just like a normal statement: subject – verb – object/other stuff:

ég tala mikið (I talk a lot)
not tala ég mikið (that would be a question or V2 main clause order).

Why is it ég tala and not something like ég talar?

tala is a regular -a verb. Present tense:

ég tala – I speak
þú talar – you speak
hann/hún/það talar – he/she/it speaks
við tölum – we speak
þið talið – you (pl.) speak
þeir/þær/þau tala – they speak

Since the subject is ég (I), the correct form is tala, not talar.

What is mikið doing here, and why does it look neuter?

mikill is an adjective meaning “much, big, great”.
The form mikið can be:

neuter singular of the adjective (e.g. mikið vatn – much water), or
an adverb meaning “a lot, much”.

In ég tala mikið, mikið is an adverb: “I talk a lot.”
As an adverb, it doesn’t agree with ég; it just modifies the verb tala.

Would a more idiomatic way to say “My mouth is dry when I talk a lot” be different in Icelandic?

Yes, a very natural way to say this is:

Ég er þurr í munninum þegar ég tala mikið.
Literally: I am dry in the mouth when I talk a lot.

Here:

Ég er þurr – I am dry
í munninum – in the mouth (dative definite)

But Munnurinn er þurr þegar ég tala mikið is still grammatically correct and understandable.

How do you pronounce the tricky letters in this sentence, like þ and ð (if present)?

In Munnurinn er þurr þegar ég tala mikið:

þ in þurr, þegar is like th in think (voiceless).
There is no ð in this sentence, only þ.
ég is commonly pronounced [jeː] or [jɛiː], roughly like “yay” but shorter/tenser.
Double consonants (like nn, rr) make the preceding vowel shorter and the consonant longer/clearer.

A rough pronunciation guide (not strict IPA):

Munnurinn ≈ “MUN-nur-in”
er ≈ “air” (short)
þurr ≈ “thurr” (with a rolled r)
þegar ≈ “THEH-gar”
ég ≈ “yay” (short)
tala ≈ “TAH-la”
mikið ≈ “MI-kith” (soft ð-like sound at the end if pronounced carefully: [ˈmɪːcɪð])

What gender and case is munnur, and how does it decline in the singular?

munnur is masculine. Singular declension:

Nominative: munnur – a mouth (subject)
Accusative: munn – (see it, open your mouth etc.)
Dative: munni – in the mouth
Genitive: munns – of a mouth

Definite forms (singular):

Nominative: munnurinn – the mouth
Accusative: munninn – the mouth (object)
Dative: munninum – in/to the mouth
Genitive: munnsins – of the mouth

In our sentence, munnurinn is nominative definite, so it’s the subject of er þurr.