Learn Hardcore Icelandic: Í borginni er loftið stundum þungt vegna mengunar. - In the city the air is sometimes heavy because of pollution.

So the structure is basically: In (the) city is (the) air sometimes heavy because‑of pollution.
Natural English: In the city, the air is sometimes heavy because of pollution.

Why is it borginni and not just borg or borgin?

Three things are happening in borginni:

Base noun: borg – city (feminine).
Definite article: borgin – the city.
In Icelandic, the definite article (the) is usually a suffix attached to the noun.
Case and number: borginni is dative singular definite of borg.

Why dative? Because of the preposition í (in).

Í takes either accusative or dative, depending on meaning.
- Accusative: movement into something – í borgina (into the city).
- Dative: location in something – í borginni (in the city).

Here we are talking about a location (where the air is heavy), so í + dative → borginni.

Why is the definite article attached (like loftið, borginni) instead of saying “the” as a separate word?

Icelandic does not use a separate word like English the in most cases. Instead, it uses a definite suffix:

loft – air
loftið – the air (neuter singular nominative definite)
borg – city
borgin – the city (feminine singular nominative definite)
borginni – in the city (feminine singular dative definite)

So where English says the city, Icelandic typically says borgin/borginni.

There is also a separate article hinn / hin / hið, but that’s used in more specific or formal contexts and not here. For everyday “the X”, you mainly see these suffixes.

Why is the word order Í borginni er loftið stundum þungt instead of putting loftið first?

Both of these are grammatically correct:

Í borginni er loftið stundum þungt vegna mengunar.
Loftið er stundum þungt í borginni vegna mengunar.

The difference is emphasis and information structure:

Sentence 1 (the original) starts with the location (In the city).
It answers a kind of “where?” first: In the city, the air is sometimes heavy…
This is often used when contrasting different places or setting a scene.
Sentence 2 starts with loftið (the air).
It sounds more like a general statement about the air, with the city mentioned later, almost as an afterthought: The air is sometimes heavy in the city…

Icelandic word order is fairly flexible, but something has to come first in the V2 slot structure (the verb usually comes second in main clauses). Here:

Í borginni is in first position.
er is the verb in second position, which is normal.
Then comes the subject loftið, followed by adverb and adjective.

Why is it þungt and not þungur or þung?

Þungur (heavy) is an adjective that agrees with the noun it describes in:

gender (masculine / feminine / neuter)
number (singular / plural)
case (nominative / accusative / dative / genitive)

Loftið (the air) is:

neuter
singular
nominative

So the adjective must also be:

neuter
singular
nominative

That form is þungt.

Rough paradigm (nominative singular, strong declension):

Masculine: þungur
Feminine: þung
Neuter: þungt

Since loftið is neuter, you must say loftið er þungt, not þungur or þung.

What is stundum, and where can it appear in the sentence?

Stundum means sometimes. It’s an adverb of frequency.

In Icelandic, adverbs like this have some flexibility in placement. All of the following are possible, with slightly different rhythm/emphasis:

Í borginni er loftið stundum þungt vegna mengunar.
Í borginni er stundum loftið þungt vegna mengunar.
Í borginni er loftið þungt stundum vegna mengunar.

The most neutral/natural version here is usually:

Í borginni er loftið stundum þungt vegna mengunar.

So: subject (loftið) → adverb (stundum) → predicate adjective (þungt).
Moving stundum too much can sound a bit marked or poetic, but it’s not ungrammatical.

What does vegna mengunar literally mean, and why is mengunar in the genitive?

vegna means because of / due to / on account of.
It always takes the genitive case.

The noun mengun (pollution) in its base form is nominative singular: mengun.
Genitive singular of mengun is mengunar.

So:

mengun – pollution (nominative)
mengunar – of pollution (genitive)

Putting it together:

vegna mengunar – because of pollution / due to pollution (literally: because-of of‑pollution).

Any time you use vegna, expect the following noun to go into genitive.

Could I say út af mengun instead of vegna mengunar? Is there a difference?

You can say both, but there is a nuance:

vegna mengunar – because of pollution / due to pollution
- More formal / neutral, often used in written language.
- Very straightforward “cause” meaning.
út af mengun – also because of pollution / due to pollution
- More colloquial, common in speech.
- Can sometimes sound a bit more like “as a result of / on account of”.

In this sentence, you could say:

Í borginni er loftið stundum þungt vegna mengunar.
Í borginni er loftið stundum þungt út af mengun.

Both are understandable and acceptable. For textbook‑style Icelandic, vegna mengunar is slightly more standard.

Why isn’t there a word like English “there,” as in “In the city there is…”?

English often uses there is/are as a dummy subject:

There is heavy air in the city.

Icelandic doesn’t need a dummy there in the same way. In this sentence:

loftið (the air) is the real grammatical subject.
er (is) is the verb.
þungt is the predicate adjective (what the air is).
Í borginni is just the location.

So the structure is closer to:

In the city, the air is sometimes heavy
rather than
In the city, there is heavy air.

If Icelandic wanted a real existential “there is” sentence, it more often uses forms like:

Það er… or Það er loft í borginni. – There is air in the city.

But that would change the meaning: we’re not saying air exists in the city; we’re saying the air (that is there) is heavy.

Can I move í borginni to the end and say Loftið er stundum þungt vegna mengunar í borginni?

Yes, this is grammatically fine:

Loftið er stundum þungt vegna mengunar í borginni.

Differences:

Original:
- Í borginni er loftið stundum þungt vegna mengunar.
- Starts with the location, gives a strong “as for the city…” feeling.
Reordered:
- Loftið er stundum þungt vegna mengunar í borginni.
- Starts with the air, and the city feels like an added specification at the end: because of pollution in the city.

Native speakers might choose one or the other depending on what they want to emphasize first: the place (city) or the thing (air). Both are correct; the original sounds slightly more like a description tied specifically to city air as opposed to air in general.

How is this sentence pronounced, roughly, for an English speaker?

Very rough guide (stressed syllables in CAPS, approximate English hints):

Í – [iː] like ee in see (long)
borginni – [ˈpɔr̥.cɪ.nɪ]
- BOR – like bor in boring, but with an unvoiced b (somewhere between p and b) and a trilled r
- gi – like gih in give (short i)
- nni – like nih-nih (short i)
er – [ɛr] like air but shorter and with a trilled r
loftið – [ˈlɔf.tɪð]
- LOFT – like English loft
- ið – like ith but with a soft ð (as in this), and a very short vowel
stundum – [ˈstʏn.tʏm]
- STUN – stun but with rounded u (close to German ü)
- dum – dum with the same rounded vowel
þungt – [θuŋkt]
- þ – like th in think
- u – like oo in book (short)
- final gt is pronounced [kt], so something like thoonkt
vegna – [ˈvɛk.na]
- VEG – like vek (short e, hard g as in get)
- na – nah
mengunar – [ˈmeŋ.gʏ.nar]
- MEN – like meng (men
  - ng)
- gu – gü (rounded, like German ü)
- nar – nar with a trilled r

Spoken smoothly:
Í BOR-gin-ni er LOFT-ið STUN-dum ÞUNGT VEG-na MEN-gu-nar.