Learn Hardcore Icelandic: Mengunin í borginni er stundum mikil og hefur áhrif á umhverfið. - The pollution in the city is sometimes great and has an effect on the environment.

Icelandic often adds a definite ending to the noun instead of using a separate word like English the. For feminine nouns like mengun, the definite nominative singular ending is -in.

mengun = pollution
mengunin = the pollution

In this sentence, mengunin is the subject, so it is in nominative definite form: the pollution (in the city) is sometimes great…

Why is it í borginni and not just í borg or í borgina?

The base noun is borg = city (feminine).

Definite article
The ending -in / -inni marks the city rather than just a city.
- borg = a city
- borgin (nom.) = the city
- borginni (dat.) = the city (in the dative case)
Case after preposition í
The preposition í (“in, into”) takes:
- dative when it means in/inside (location)
- accusative when it means into (movement)

Here it is location (in the city), so í requires the dative:

í borginni = in the city (dative, definite)

í borgina would be accusative and would suggest motion into the city, e.g. Ég fer í borgina – I’m going into the city.

What case is borginni, and how is that formed from borg?

Borginni is dative singular definite.

From the base noun borg (feminine):

Nominative indefinite: borg – a city
Nominative definite: borgin – the city
Dative indefinite: borg – (to/in) a city
Dative definite: borginni – (to/in) the city

So borg + dative + definite = borginni.
Because í (with location) requires dative, we get í borginni.

Why is it mikil and not mikill or mikið?

The adjective must agree with the noun in gender, number, and case.

Noun: mengunin – feminine, singular, nominative, definite
Adjective stem: mikill – great, large, a lot of

For feminine nominative singular, the strong adjective ending is -il:

masculine nom. sg.: mikill
feminine nom. sg.: mikil
neuter nom. sg.: mikið

Since mengunin is feminine singular nominative, we use mikil to match:

Mengunin … er … mikil = The pollution … is great / high.

Where does stundum usually go in the sentence, and could I move it?

Stundum means sometimes and is an adverb. Icelandic adverbs are fairly flexible.

Your sentence:

Mengunin í borginni er stundum mikil…

Possible alternatives (all natural, but with slightly different emphasis):

Stundum er mengunin í borginni mikil…
(Sometimes, the pollution in the city is high… — stronger emphasis on sometimes.)
Mengunin í borginni stundum er mikil…
This is unusual and sounds marked or poetic; not typical spoken Icelandic.

Two especially common versions are:

Mengunin í borginni er stundum mikil…
Stundum er mengunin í borginni mikil…

So yes, you can move stundum toward the beginning of the clause, but not just anywhere.

Why is the verb er needed? Could we say Mengunin í borginni stundum mikil without it?

You cannot drop er here. Icelandic, unlike Russian or Arabic, requires a form of the verb vera (to be) in the present tense in normal sentences.

So:

Correct: Mengunin í borginni er stundum mikil.
Incorrect: Mengunin í borginni stundum mikil.

You always need er (or another form of vera) to link a subject to an adjective or noun in present-tense statements:
X er Y – X is Y.

Why is it hefur and not hafa or á for “has”?

The verb hafa means to have. Its present forms include:

ég hef – I have
þú hefur – you (sg.) have
hann / hún / það hefur – he / she / it has
við höfum – we have
þið hafið – you (pl.) have
þeir / þær / þau hafa – they have

The subject is mengunin (the pollution), which is 3rd person singular, so we use hefur:

Mengunin … hefur áhrif… – The pollution … has an effect…

Why not á?
Á is also a verb (to own, to have in some contexts), but it is not used in the fixed expression hafa áhrif á e-ð (“to have an effect on something”). That phrase always uses hafa.

What exactly does the phrase hafa áhrif á mean, and is áhrif plural?

Hafa áhrif á is an idiomatic expression meaning to have an effect on / to influence.

hafa = to have
áhrif = effect(s), influence
á
- accusative = on, onto

So hefur áhrif á umhverfið = has an effect on the environment / affects the environment.

About áhrif:

Formally it is a neuter plural noun (it has no singular form in normal use).
But in meaning it often behaves like an uncountable mass noun (“effect, influence”).

In this sentence, áhrif is the direct object of hefur:

subject: mengunin
verb: hefur
object: áhrif
prepositional phrase: á umhverfið

Why is it á umhverfið and not á umhverfinu?

Both á and í can take different cases depending on meaning.

For á:

dative = location on/at: on, on top of, at
accusative = direction or impact onto: onto, to, affecting

The structure hafa áhrif á e-ð (“have an effect on something”) always uses á with the accusative, because the thing is being affected or acted upon.

umhverfið is accusative singular definite (neuter):
- umhverfi = environment
- umhverfið = the environment (nom./acc. definite)

So:

á umhverfið = on the environment (as an object of influence)
á umhverfinu would be dative and would fit meanings like on the environment in the sense of location (placed on), which doesn’t match this idiom.

Therefore, hefur áhrif á umhverfið is the correct idiomatic and grammatical form.

Why does umhverfið also have a definite ending, like mengunin and borginni?

Icelandic very often uses the definite form where English also uses the, and sometimes even where English might say just environment in a general sense.

umhverfi = environment
umhverfið = the environment (nominative/accusative definite)

In this sentence, we are talking about the environment in general as something that is affected. Icelandic tends to prefer the definite form here, so á umhverfið is natural and idiomatic.

All three nouns are definite here:

mengunin – the pollution
borginni – in the city
umhverfið – the environment

Can I change the word order to Stundum er mengunin í borginni mikil og hefur áhrif á umhverfið?

Yes, that is fully correct and very natural.

Two common word orders:

Mengunin í borginni er stundum mikil og hefur áhrif á umhverfið.
Neutral emphasis; starts with the topic the pollution in the city.
Stundum er mengunin í borginni mikil og hefur áhrif á umhverfið.
Slightly stronger emphasis on sometimes; stylistically very common.

Icelandic allows some flexibility in word order, especially with adverbs like stundum, as long as the verb stays in the second position of the clause (the V2 rule). In Stundum er mengunin…, stundum is first and er is second, so it obeys that rule.

How do you pronounce mengunin, especially the -gn- and final -in?

Pronunciation (approximate):

men- like men in English
-gu- like guh (short u as in put)
-nin like nin (short i as in sit)

Key points:

In mengun, the gn is usually pronounced close together, often like [ŋk] or [ŋg] depending on accent, but learners can safely pronounce a clear g
- n and still be understood.
The u is short, not like the English long oo.
The final -in is one syllable, with a clear n: -nin, not like English neen.

So roughly: MEN-gu-nin (with all vowels short).

How do you pronounce the ð at the end of umhverfið?

The letter ð in Icelandic is usually pronounced like the th in English this or that (voiced th sound).

In umhverfið:

um-hver-fiþ (approximate) — the final ð is a soft voiced th.
It is often quite weak in fast speech, but as a learner you can pronounce a clear voiced th at the end: um-hver-vith.

So the important part: ð ≈ English th in this, not like English d.