Gangurinn í nýju íbúðinni er mjór, en mjög bjartur.

The ending -inn is the definite article attached to the noun.

• gangur = hallway / corridor
• gangurinn = the hallway

Icelandic usually puts the at the end of the noun instead of using a separate word like English the.

So here:

• gangur → indefinite
• gangurinn → definite

Because the sentence is talking about a specific hallway, it uses gangurinn.

Because the preposition í can require different cases, and here it means in as a location, so it takes the dative.

This phrase means in the new apartment, and both the adjective and the noun have to match the required case:

• í
  • dative for location
• íbúðinni = dative singular definite of íbúð (apartment)
• nýju = adjective form that agrees with íbúðinni

So:

• í íbúðinni = in the apartment
• í nýju íbúðinni = in the new apartment

If the meaning were motion into something, Icelandic often uses accusative instead.

Because it has to agree with íbúðinni.

Here, íbúð is:

• feminine
• singular
• dative
• definite

So the adjective nýr (new) must appear in the matching form, which is nýju.

This is one of the big features of Icelandic: adjectives change form depending on the noun they describe.

In this sentence:

• dictionary form: nýr
• actual form used: nýju

Because they are predicate adjectives describing the subject gangurinn.

After er (is), Icelandic adjectives still agree with the subject. The subject here is:

• gangurinn
• masculine
• singular
• nominative

So the adjectives also appear in masculine singular nominative:

• mjór = narrow
• bjartur = bright

That is why you get:

• Gangurinn ... er mjór
• Gangurinn ... er ... bjartur

They are both describing gangurinn.

Because different adjectives belong to different declension patterns.

Even when two adjectives are agreeing with the same noun, they do not always look identical in form. Both are masculine singular nominative here, but their dictionary forms and patterns are different:

• mjór already has the masculine nominative singular form mjór
• bjartur has the masculine nominative singular form bjartur

So they are matching the same grammar, just with different adjective patterns.

This is normal in Icelandic and something you gradually get used to.

Mjög means very. It is an adverb, and adverbs do not agree with nouns the way adjectives do.

So:

• mjög bjartur = very bright

Unlike nýju, mjór, and bjartur, the word mjög stays the same. It does not change for gender, number, or case.

Because its position shows that it modifies only bjartur.

So the sentence means:

• narrow, but very bright

not

• very narrow and very bright

If you wanted both adjectives to be intensified, you would normally repeat the adverb or structure the sentence differently.

En means but.

It connects two contrasting descriptions of the hallway:

• mjór = narrow
• mjög bjartur = very bright

So the sentence is saying that the hallway has one negative or limiting quality and one positive or contrasting quality.

Because it identifies which hallway we are talking about.

The structure is basically:

• Gangurinn í nýju íbúðinni = the hallway in the new apartment
• er mjór, en mjög bjartur = is narrow, but very bright

So the prepositional phrase belongs with the noun phrase and narrows it down:

not just any hallway, but the hallway in the new apartment.

It is fairly similar to English in this sentence.

The pattern is:

• subject
• extra phrase describing the subject
• verb
• adjective(s)

So:

• Gangurinn í nýju íbúðinni = subject phrase
• er = is
• mjór, en mjög bjartur = description

Icelandic word order can be more flexible than English, but this sentence uses a very straightforward, neutral order.

The comma helps mark the contrast before en (but).

In a sentence like this, it makes the pause and opposition clearer:

• mjór, en mjög bjartur

You can think of it as separating the two descriptive parts. In writing, this is a very natural way to present the contrast.

In this sentence, bjartur is best understood as bright in the sense of well-lit or full of light.

So for a hallway, it does not mean bright like a bright color. It means the hallway has a lot of light, probably from windows, open space, or good lighting.

That is a very common use of bjartur for rooms and indoor spaces.

They are:

• gangur = hallway, corridor
• nýr = new
• íbúð = apartment, flat
• mjór = narrow
• bjartur = bright

In the sentence, they appear in inflected forms:

• gangurinn ← gangur
• nýju ← nýr
• íbúðinni ← íbúð
• mjór ← same form here
• bjartur ← same masculine nominative singular form here

This is a good example of how Icelandic changes words depending on their role in the sentence.