Loftið í borginni er hreint í dag.

The ending -ið is the definite article attached to the noun loft (air).

• loft = air (indefinite)
• loftið = the air (definite)

In Icelandic, the definite article (the equivalent of English the) is usually not a separate word before the noun. Instead, it is added to the end of the noun as a suffix:

• stóll = chair → stóllinn = the chair
• hús = house → húsið = the house
• loft = air → loftið = the air

So loftið specifically means the air, not just air in general.

Borginni is the noun borg (city) in the dative singular, definite form.

Breakdown:

• Basic form: borg = city (nominative, indefinite)
• Gender: feminine
• Dative singular (indefinite) of borg: borg → borg (some feminine nouns don’t change form visibly here)
• Add the definite article in the dative singular feminine: -inni

So:

• borg = a city
• borginni = in the city / the city (as an indirect object, location, etc.)

In this sentence, í borginni = in the city, with í (in) requiring the dative case for a static location, so borginni must be in the dative.

The preposition í can take dative or accusative, depending on meaning:

• Dative → location (where something is)
• Accusative → motion/direction (where something is going)

In your sentence, the meaning is location: the air is clean in the city (not moving into it).
So you use dative: í borginni.

Compare:

• Ég bý í borginni. = I live in the city. (location → dative)
• Ég fer í borgina. = I am going to the city. (movement → accusative)

So í borginni is correct here because it describes where the air is, not movement.

Icelandic adjectives must agree with the noun they describe in:

• gender (masculine / feminine / neuter)
• number (singular / plural)
• case (nominative / accusative / etc.)

The subject here is loftið (the air):

• loft is neuter
• here it is singular
• and in the nominative as the subject

So the adjective hreinn (clean) must be in neuter, singular, nominative:

• Masculine: hreinn
• Feminine: hrein
• Neuter: hreint

Therefore: Loftið … er hreint …
Hreint matches loftið (neuter singular nominative).

The grammatical subject is loftið (the air).

Structure:

• Loftið í borginni = the air in the city (subject phrase)
• er = is (verb)
• hreint = clean (predicative adjective, describing the subject)
• í dag = today (time expression)

The adjective hreint agrees with loftið in gender, number and case:

• loftið: neuter, singular, nominative
• hreint: neuter, singular, nominative (to match loftið)

Í borginni and í dag are prepositional/time phrases; hreint does not agree with those, only with the subject loftið.

Yes, Icelandic word order is more flexible than English. Both are grammatically correct:

• Loftið í borginni er hreint í dag.
• Í dag er loftið í borginni hreint.

The basic rule is that the finite verb (here er) normally stays in second position in main clauses. So you can move í dag to the front for emphasis:

• Í dag (1st position)
• er (2nd position – verb)
• loftið í borginni hreint (rest of the sentence)

Subtle differences in emphasis:

• Loftið í borginni er hreint í dag.
  – Neutral; just stating a fact about today.

• Í dag er loftið í borginni hreint.
  – Emphasis on today (maybe in contrast with other days).

But both are natural sentences.

Í dag is flexible in position. Common options:

1. At the end (very typical, neutral):

   • Loftið í borginni er hreint í dag.

2. At the beginning for emphasis on today:

   • Í dag er loftið í borginni hreint.

3. Directly after the verb (also possible, but less neutral here):

   • Loftið í borginni er í dag hreint.
     (grammatically okay, but sounds a bit marked or stylistic)

You cannot just drop it into the middle randomly; you should still respect:

• verb in second position (if í dag is first)
• keeping related parts together (subject phrase, verb, complement)

The most natural two versions here are:

• Loftið í borginni er hreint í dag.
• Í dag er loftið í borginni hreint.

Yes, you can. That sentence is grammatically correct:

• Loftið í borginni er hreint. = The air in the city is clean.

You just lose the time limitation – without í dag, it sounds more like a general statement about the air in the city, rather than specifically about today.

So:

• With í dag: clean today (maybe not always).
• Without í dag: generally clean, as a more permanent or typical fact.

Er is the present tense, 3rd person singular form of the verb vera (to be). It is not always translated as is; it can also correspond to am / are depending on the subject.

Present tense of vera:

• ég er = I am
• þú ert = you are (singular)
• hann/hún/það er = he/she/it is
• við erum = we are
• þið eruð = you are (plural)
• þeir/þær/þau eru = they are

So er can be:

• I am → ég er
• It is → það er
• The air is → loftið er

In your sentence, er corresponds to is because the subject loftið is like it in English.

In Icelandic, every noun has a grammatical gender: masculine, feminine, or neuter.

• loft is neuter
• borg is feminine

Often, you simply need to learn the gender with the noun, because it is not always predictable from meaning. There are some patterns, but many exceptions.

Practical advice:

• Always learn nouns like this:
  • loft (hk.) – hk. = hvorugkyn (neuter)
  • borg (kvk.) – kvk. = kvenkyn (feminine)
• Then you will know which adjective forms and article endings to use:
  • loftið er hreint (neuter: -ið, hreint)
  • borgin er stór (feminine: -in, stór)

Approximate pronunciation (IPA):

• Loftið → [ˈlɔftɪð]
• í → [iː]
• borginni → [ˈpɔr̥cɪnɪ] (the b is often devoiced, sounding close to p)
• er → [ɛr]
• hreint → [r̥ei̯nt]
• í → [iː]
• dag → [taɣ] or [taːɣ] (the g is a soft, fricative sound)

Tricky points:

• ð in loftið: like the th in this, but often very soft.
• hr- in hreint: the h is pronounced before the r, giving a breathy, voiceless r.
• Final -g in dag: not like English g in dog, more like a soft gh or voiced ch in the back of the throat.
• Stress: usually on the first syllable of each word: LOFT-ið, BORG-inni, HREINT, DAG.

Said smoothly and naturally, it sounds roughly like:

• LOFT-ið í BORG-inni er HREINT í DAG.