Na stepeništu je mokar pod, zato hodamo oprezno.

Because na often means on / at when talking about location, and after a location meaning it takes the locative case.

• stepenište = staircase / stairwell / stairs area
• locative singular of stepenište is stepeništu

So:

• na stepeništu = on the staircase / in the stairwell

A useful contrast:

• na stepenište = onto the staircase (motion toward it, accusative)
• na stepeništu = on the staircase (location, locative)

They are related, but not always identical.

• stepenište usually means the staircase area, stairwell, or the whole stair structure
• stepenice means the stairs / steps themselves

In many everyday situations, both can sound natural, depending on what you want to emphasize.

For example:

• Na stepeništu je mokar pod. = The floor in the staircase/stairwell is wet.
• Na stepenicama je mokro. = The stairs are wet.

Your sentence focuses on the floor surface in that area, so stepeništu works well.

Both are grammatically correct. The difference is mostly word order and emphasis.

Croatian word order is more flexible than English.

• Na stepeništu je mokar pod sounds natural and introduces the situation first: On the staircase, the floor is wet
• Na stepeništu pod je mokar is possible, but less natural here
• Pod je mokar na stepeništu shifts the focus more toward the floor

In your sentence, je mokar pod is a very normal way to present new information after the location phrase.

Because mokar is the normal adjective form here: masculine singular nominative, agreeing with pod.

• pod = masculine singular noun
• mokar = masculine singular adjective

So:

• mokar pod = a wet floor / the floor is wet

You may also encounter mokri in other contexts, but in this sentence mokar is the standard form.

Since the adjective is part of the predicate after je, Croatian uses the basic agreeing form:

• pod je mokar = the floor is wet

Because Croatian does not have articles like English a and the.

So pod can mean:

• a floor
• the floor

The exact meaning depends on context.

In this sentence, English naturally translates it as the floor because the situation is specific:

• Na stepeništu je mokar pod = The floor on the staircase is wet

Here zato means therefore, so, or that’s why.

It connects the cause and the result:

• Na stepeništu je mokar pod = The floor on the staircase is wet
• zato hodamo oprezno = so we walk carefully

It is a very common linking word in Croatian.

Similar English meanings:

• so
• therefore
• that is why

Because zato is connecting two clauses:

• Na stepeništu je mokar pod
• zato hodamo oprezno

In Croatian, a comma is normally used before this kind of linking word when it introduces a result or conclusion.

So the punctuation in your sentence is standard and natural.

Because hodati means to walk, while ići means to go.

In this sentence, the speaker is talking about the manner of movement:

• hodamo oprezno = we walk carefully

That sounds more precise than idemo oprezno, because the point is that people are physically walking carefully on a slippery surface.

Compare:

• Idemo kući. = We are going home.
• Hodamo polako. = We are walking slowly.

Because oprezno is an adverb, and it modifies the verb hodamo.

• oprezan / oprezna / oprezno can be adjective forms depending on gender
• oprezno as an adverb means carefully / cautiously

So:

• hodamo oprezno = we walk carefully

If you said oprezni, that would be an adjective meaning careful referring to people, for example:

• Mi smo oprezni. = We are careful.

But here you need an adverb because it describes how we walk.

Hodamo is:

• 1st person plural
• present tense

It means we walk or we are walking, depending on context.

Croatian present tense often covers both English ideas:

• habitual: we walk
• current action: we are walking

Here it most naturally means something like we walk carefully or we’re walking carefully because of the wet floor.

Yes. Croatian often allows several natural versions. For example:

• Na stepeništu je pod mokar, zato hodamo oprezno.
• Na stepenicama je mokro, zato hodamo oprezno.
• Pod na stepeništu je mokar, zato hodamo oprezno.

Your original sentence is perfectly natural, but the exact version may change depending on what the speaker wants to emphasize:

• the location
• the floor
• the general fact that it is wet on the stairs

Yes, pod is fine here.

Pod means the floor surface, and in a building’s staircase or stairwell there can be flat floor areas as well as steps. So if the speaker is referring to the surface in that stair area, pod is natural.

If you want to emphasize the steps themselves, Croatian would more likely use:

• stepenice = stairs / steps

So there is a small difference:

• mokar pod = wet floor surface
• mokre stepenice = wet stairs / steps

Your sentence suggests that the walking surface in the staircase area is wet, which is why people are being careful.