Ich gehe die Treppe langsam hinunter, weil der Boden nass ist.

Because die Treppe hinuntergehen uses the staircase as the route/path of movement, not as a location.

• auf der Treppe = on the stairs / located on the staircase
• die Treppe hinuntergehen = go down the stairs

Here, die Treppe is in the accusative because it functions like a path being traversed. German often does this with movement expressions:

• den Berg hinaufgehen = go up the mountain
• die Straße entlanggehen = walk along the street
• den Fluss entlangfahren = drive along the river

So die Treppe is not a normal direct object in the English sense; it is the route of motion.

Yes. Hinuntergehen is a separable verb.

In a main clause, the conjugated verb goes to position 2, and the separable prefix moves to the end:

• Ich gehe die Treppe langsam hinunter.

Base form:

• hinuntergehen

Split in the sentence:

• gehe ... hinunter

This is the same pattern as:

• Ich stehe früh auf. from aufstehen
• Er kommt morgen zurück. from zurückkommen

If the verb is in a subordinate clause, the parts stay together:

• ..., weil ich die Treppe langsam hinuntergehe.

These all mean down, but there are small differences:

• hinunter = down, usually away from the speaker/reference point
• herunter = down, usually toward the speaker/reference point
• runter = shorter, more colloquial form of herunter/hinunter

In everyday speech, many Germans do not keep the hin- / her- distinction very strictly, especially with common motion verbs. So you may hear:

• die Treppe hinuntergehen
• die Treppe heruntergehen
• die Treppe runtergehen

All are common. Hinunter sounds a bit more neutral or careful in standard German; runter is more informal.

Because weil is a subordinating conjunction. In German, subordinating conjunctions usually send the finite verb to the end of the clause.

So:

• main clause: Der Boden ist nass.
• after weil: ..., weil der Boden nass ist.

This is a very important German word-order rule.

Other common subordinating conjunctions that do the same thing:

• dass
• wenn
• obwohl
• bevor
• als

Examples:

• ..., weil ich müde bin
• ..., obwohl er krank ist
• ..., dass sie heute kommt

Because der Boden is the subject of the clause weil der Boden nass ist.

The verb sein works like to be in English. After sein, the subject stays in the nominative:

• Der Boden ist nass.
• Das Wasser ist kalt.
• Die Treppe ist steil.

So:

• der Boden = nominative subject
• nass = predicate adjective
• ist = verb

If it were den Boden, that would be accusative, which would not fit here.

Because nass is a predicate adjective here, not an adjective directly in front of a noun.

After verbs like sein, werden, and bleiben, adjectives usually do not take endings:

• Der Boden ist nass.
• Das Kind ist müde.
• Die Straße wird glatt.

But when an adjective comes before a noun, it does take an ending:

• der nasse Boden
• eine nasse Straße
• die müde Frau

So:

• der Boden ist nass = no ending
• der nasse Boden = adjective ending required

Yes, Ich gehe langsam die Treppe hinunter is also possible.

German word order in the middle of the clause is flexible, especially with adverbs. In your sentence:

• Ich gehe die Treppe langsam hinunter

the focus is first on the path (die Treppe), then on the manner (langsam).

If you say:

• Ich gehe langsam die Treppe hinunter

the adverb langsam is brought earlier, so the slowness is a bit more prominent.

Both sound natural. German often allows this kind of variation, with small changes in emphasis rather than a big change in meaning.

Because in German, subordinate clauses are normally separated by a comma.

So the sentence is divided into:

• main clause: Ich gehe die Treppe langsam hinunter
• subordinate clause: weil der Boden nass ist

That is why you write:

• Ich gehe die Treppe langsam hinunter, weil der Boden nass ist.

This comma is required in standard German.

Yes, but the word order changes.

With weil, the verb goes to the end:

• Ich gehe die Treppe langsam hinunter, weil der Boden nass ist.

With denn, the clause keeps normal main-clause word order:

• Ich gehe die Treppe langsam hinunter, denn der Boden ist nass.

Both can mean because, but there is a difference in grammar:

• weil = subordinating conjunction → verb at the end
• denn = coordinating conjunction → normal word order

In everyday speech, weil is often more common.

Yes. Gehen is completely normal here.

German often uses gehen for movement on foot, including stairs:

• die Treppe hinaufgehen
• die Treppe hinuntergehen
• nach Hause gehen

You could also hear other verbs in some contexts:

• laufen = walk/run, depending on context
• steigen = climb/step, sometimes used with stairs, but often sounds a bit more specific or formal

For a simple neutral statement, die Treppe hinuntergehen is the most straightforward choice.