Garderobestangen monteres i skapstammen.

Because Norwegian very often makes compound nouns by joining words together.

garderobestangen =

• garderobe = wardrobe / clothes storage
• stang = rod / bar
• -en = the

So garderobestangen means the wardrobe rod or the clothes rail.

This is very normal in Norwegian. Where English often uses separate words, Norwegian often combines them into one:

• bokhylle = bookshelf
• spisebord = dining table
• garderobestang = wardrobe rod

The ending -en marks the noun as definite singular: the wardrobe rod.

The basic noun is:

• en garderobestang = a wardrobe rod

Definite form:

• garderobestangen = the wardrobe rod

This is a key difference from English. In Norwegian, definiteness is often shown by an ending on the noun instead of a separate word like the.

Here monteres means is installed, is mounted, or is assembled/fitted, depending on context.

It comes from the verb å montere = to mount / install / assemble.

So:

• monterer = installs / is installing
• monteres = is installed / is mounted

In an instruction manual, monteres is very common because it focuses on what is done to the object, not on who does it.

Because monteres is the s-passive form.

Compare:

• Man monterer garderobestangen i skapstammen.
  = One installs the wardrobe rod in the cabinet frame.

• Garderobestangen monteres i skapstammen.
  = The wardrobe rod is installed in the cabinet frame.

So -s here makes the verb passive. This is especially common in:

• instructions
• manuals
• formal writing
• product descriptions

It is similar to English is mounted / is installed.

Grammatically, it looks like a present-tense passive sentence, but in context it is usually understood as an instruction-manual style statement.

So even though it literally looks like:

• The wardrobe rod is installed in the cabinet frame

in a manual it often really means something like:

• Install the wardrobe rod in the cabinet frame
• The wardrobe rod should be mounted in the cabinet frame

Norwegian instructions often use this style instead of a direct command.

Skapstammen means something like the cabinet frame, the carcass of the cabinet, or the main structure of the wardrobe/cabinet.

It breaks down as:

• skap = cabinet / cupboard / wardrobe
• stamme = frame / main structure
• -n (in stammen) = the

So skapstammen is not just any cabinet-related thing. It specifically refers to the main body/frame of the cabinet.

Because i means in/inside/within, and that fits the idea of mounting the rod inside the cabinet structure.

• i = in / inside
• på = on / onto / on the surface of

A wardrobe rod is usually installed inside the cabinet frame, not simply on top of it or on its surface, so i is the natural preposition here.

Because the sentence is talking about specific parts in a known context, usually a particular piece of furniture being assembled.

So:

• garderobestangen = the wardrobe rod
• skapstammen = the cabinet frame

In assembly instructions, this is very common. The manual assumes you know which rod and which cabinet frame it means, because they are the parts of the product you are currently working with.

Yes. Norwegian has more than one way to form the passive.

Two common options are:

• Garderobestangen monteres i skapstammen.
• Garderobestangen blir montert i skapstammen.

Both can mean The wardrobe rod is installed in the cabinet frame.

The difference is mainly style:

• monteres is shorter and very common in manuals and formal instructions
• blir montert can sound a little less compact and sometimes a bit more like ordinary prose

In instruction texts, monteres is especially typical.

Because Norwegian usually puts definiteness on the noun itself.

English:

• the rod
• the frame

Norwegian:

• stangen
• stammen

This is one of the most important differences for English speakers learning Norwegian.

You do sometimes also get a separate definite word, but usually when there is an adjective:

• stangen = the rod
• den lange stangen = the long rod

So in your sentence, no separate the is needed.

The dictionary forms are:

• en garderobestang = a wardrobe rod
• en skapstamme = a cabinet frame / cabinet carcass
• å montere = to mount / install / assemble

The sentence uses definite noun forms and a passive verb form:

• garderobestangen
• skapstammen
• monteres

No, this is normal Norwegian sentence structure.

• Garderobestangen = subject
• monteres = verb
• i skapstammen = prepositional phrase

So the pattern is basically:

Subject + verb + prepositional phrase

That is very straightforward Norwegian word order for a main clause.

A more direct imperative version could be:

• Monter garderobestangen i skapstammen.

That means:

• Install the wardrobe rod in the cabinet frame.

This is more like a direct instruction to the reader.
The original sentence, Garderobestangen monteres i skapstammen, is a bit more impersonal and is very typical of manuals.