Ég geng niður stigann.

Geng is the 1st person singular present tense form of the verb ganga, which means to walk or to go on foot.

So:

• að ganga = to walk
• ég geng = I walk / I am walking

This verb is a bit irregular, so the stem changes:

• ganga → geng

That is why you do not see ég ganga here.

In normal Icelandic, the subject pronoun is usually kept:

• Ég geng = I walk

Even though the verb form gives some information about the person, Icelandic does not usually drop subject pronouns the way some languages do. So Ég geng is the normal, natural way to say it.

Here niður means down and shows direction of movement.

So the sentence has the idea:

• Ég geng = I walk
• niður = down
• stigann = the stairs / the staircase

Together: I walk down the stairs / I am walking down the stairs.

In Icelandic, words like niður, upp, inn, and út are very common for expressing movement and direction.

This is a very common and important distinction in Icelandic:

• niður = downward, showing movement
• niðri = down below, showing location

So:

• Ég geng niður stigann = I walk down the stairs.
  (movement)
• Ég er niðri = I am downstairs/down below.
  (location)

Icelandic often uses one form for direction and another for position.

Stigann is the definite accusative singular form of stigi.

Base noun:

• stigi = stairs, staircase, stairway, ladder (depending on context)

In this sentence it means the stairs or the staircase.

The ending -ann shows that it is:

• definite = the
• accusative singular

So:

• stigi = a staircase / stairs
• stigann = the staircase / the stairs

Because this phrase expresses movement along a path / in a direction.

In niður stigann, the noun after the directional word appears in the accusative. This is a common pattern with motion expressions in Icelandic.

So stigann is not a normal direct object in the English sense. Instead, it is part of a directional phrase meaning something like:

• down the stairs
• down the staircase

A learner can think of it as: movement through/along/down a route often uses the accusative.

Because Icelandic and English package this idea differently.

English often uses the plural:

• the stairs

But Icelandic often uses the singular noun:

• stiginn / stigann
• literally more like the staircase or the stairway

So even if the English translation uses stairs, the Icelandic singular is perfectly natural.

This is the normal way to build this kind of directional expression in Icelandic:

• niður stigann = down the stairs
• upp stigann = up the stairs

The directional word comes first, then the noun phrase showing the route or path.

So the sentence structure is:

• Ég = subject
• geng = verb
• niður stigann = directional phrase

That makes Ég geng niður stigann a very natural word order.

Yes. Icelandic present tense often covers both the simple present and the present progressive that English distinguishes.

So this sentence can mean:

• I walk down the stairs
• I am walking down the stairs

Which one is intended depends on context.

Usually ganga means walk, especially going on foot. In many contexts, though, it can also be used in ways that feel a bit broader than English walk.

In this sentence, the meaning is clearly physical movement on foot:

• Ég geng niður stigann = I walk down the stairs

So for this example, walk is the best core meaning to remember.