Stigagangurinn er langur, svo íbúarnir ganga hægt niður hann.

It is the noun stigagangur plus the suffixed definite article.

• stigagangur = stairwell / staircase area
• -inn = the

So stigagangurinn means the stairwell. It is masculine singular nominative here because it is the subject of the first clause.

Because langur is a predicate adjective after er (is).

In Icelandic, adjectives used after verbs like vera usually appear in the strong form, not the weak definite form. So even though the noun is definite, you say:

• Stigagangurinn er langur.

not

• Stigagangurinn er langi.

The adjective still agrees with the noun in gender, number, and case:

• stigagangurinn = masculine singular nominative
• langur = masculine singular nominative

Here svo means so or therefore. It connects the two clauses:

• Stigagangurinn er langur
• svo íbúarnir ganga hægt niður hann

So it is working as a coordinating conjunction, not as an adverb meaning something like then or thus.

Because in this sentence svo is functioning as a conjunction linking two main clauses. After a coordinating conjunction, Icelandic normally keeps regular main-clause order:

• íbúarnir ganga

If you used an adverbial connector such as þess vegna (therefore), then you would usually get verb-second order:

• Þess vegna ganga íbúarnir hægt niður hann.

So the lack of inversion after svo is normal here.

The base noun is íbúi, meaning resident or inhabitant.

Its forms here are:

• íbúi = a resident
• íbúar = residents
• íbúarnir = the residents

So -nir here is part of the definite plural ending. Icelandic usually puts the on the end of the noun instead of using a separate word.

Because the subject is íbúarnir, which is third person plural, and the present-tense form of ganga for they is ganga.

A quick comparison:

• ég geng = I walk
• þú gengur = you walk
• hann/hún gengur = he/she walks
• við göngum = we walk
• þið gangið = you all walk
• þeir/þær ganga = they walk

So íbúarnir ganga = the residents walk.

A small thing that often confuses learners: the infinitive is also ganga, so here the finite verb happens to look exactly like the dictionary form.

Here hægt is being used adverbially, meaning slowly.

It comes from the adjective hægur = slow. Icelandic often uses the neuter singular form of an adjective as an adverb:

• hægur = slow
• hægt = slowly / in a slow way

So ganga hægt means walk slowly.

Because the pronoun refers back to stigagangurinn, and stigagangur is a masculine noun.

So the pronoun must match that grammatical gender:

• stigagangur → hann

Not það, because það would be for a neuter noun.

In this expression, niður is used with motion, so the object is understood in the accusative, and hann is the correct form here.
Also, for masculine singular, hann is both nominative and accusative, so the form does not visibly change.

Because niður and the pronoun belong together as a directional phrase: down it.

In Icelandic, words like niður, upp, inn, út often stay close to the verb and come before the noun or pronoun they govern:

• ganga niður hann
• fara inn í húsið
• hlaupa upp stigann

So niður hann is the natural order here.

Yes, absolutely.

You could say:

• Stigagangurinn er langur, svo íbúarnir ganga hægt niður stigaganginn.

That is grammatically fine. Using hann simply avoids repeating stigagangurinn. Icelandic uses pronouns this way very naturally when the reference is clear.

Because the sentence contains two full clauses:

• Stigagangurinn er langur
• íbúarnir ganga hægt niður hann

The comma helps separate them clearly. In Icelandic writing, a comma before a conjunction like svo is very normal when it joins two independent clauses.