Ermelengden er ikke kort, men tvert imot lang.

Ermelengden is a compound noun:

• erme = sleeve
• lengde = length
• ermelengde = sleeve length

Then -n / -en makes it definite:

• ermelengde = sleeve length
• ermelengden = the sleeve length

So Ermelengden er ... means The sleeve length is ...

Because it is in the definite form.

In Norwegian, nouns usually show definiteness by adding the article to the end of the noun:

• en bil = a car
• bilen = the car

The same thing happens here:

• ermelengde = sleeve length
• ermelengden = the sleeve length

So instead of using a separate word like the, Norwegian often attaches definiteness to the noun itself.

Because Norwegian normally expresses definiteness with a suffix on the noun, not with a separate article.

English:

• the sleeve length

Norwegian:

• ermelengden

That final -en already contains the idea of the.

Because in a normal Norwegian main clause, the finite verb usually comes in second position.

Here the structure is:

• Ermelengden = subject
• er = finite verb
• ikke = negation
• kort = adjective

So:

• Ermelengden er ikke kort.

This is the normal order for a main clause.
Compare:

• Han er ikke hjemme. = He is not home.
• Boka er ikke ny. = The book is not new.

Tvert imot means on the contrary or quite the opposite.

It is used to strongly reverse what was just said or denied.

So:

• ikke kort, men tvert imot lang = not short, but on the contrary long

It adds emphasis. Without tvert imot, the sentence would still make sense:

• Ermelengden er ikke kort, men lang.

But tvert imot makes the contrast stronger and more explicit.

Men means but.

It connects two contrasting ideas:

• ikke kort = not short
• lang = long

So the sentence contrasts the two descriptions:

• Ermelengden er ikke kort, men tvert imot lang.
• The sleeve length is not short, but on the contrary long.

Because lang agrees with the noun it describes, and here the noun is singular common gender.

Predicate adjectives after å være often agree with the noun:

• common gender singular: lang
• neuter singular: langt
• plural: lange

So:

• Ermelengden er lang.
  because ermelengden is not neuter

If the noun were neuter, you would use langt:

• Ermet er langt. = The sleeve is long.

Because kort is simply the correct form of that adjective here. The adjective kort is a little less visibly changeable than lang.

For kort:

• common gender singular: kort
• neuter singular: kort
• plural: korte

So both of these are correct:

• Ermelengden er kort.
• Ermet er kort.

By contrast, lang changes more obviously:

• Ermelengden er lang.
• Ermet er langt.

Norwegian often leaves out repeated words when they are easy to understand from context.

Full version:

• Ermelengden er ikke kort, men er tvert imot lang.

More natural and concise:

• Ermelengden er ikke kort, men tvert imot lang.

This kind of omission is very common and sounds natural.

Yes. That is completely grammatical and natural.

• ... men lang = ... but long
• ... men tvert imot lang = ... but on the contrary long

The version with tvert imot is more emphatic. It stresses that the opposite is true, not just a different description.

Yes, it is a normal and understandable compound, especially in contexts like clothing, tailoring, online shopping, or product descriptions.

Norwegian often makes compound nouns very freely, just like English:

• bordlengde = table length
• armlengde = arm length
• ermelengde = sleeve length

So even if a learner has not seen the exact word before, it is very natural once you know the parts.

In some cases, feminine nouns in Bokmål can take either -en or -a in the definite singular, depending on style and preference.

Since lengde is a feminine noun, you may sometimes encounter forms like:

• ermelengden
• ermelengda

However, ermelengden is completely standard Bokmål and very common, especially in more neutral or formal written language.

It is natural, but it sounds a little careful or descriptive, as you might find in:

• clothing descriptions
• fitting advice
• product reviews
• measurements

The phrase tvert imot also adds a slightly more formal or emphatic tone than simple everyday speech.

A more casual version might be:

• Ermelengden er ikke kort, den er ganske lang.
• Ermelengden er ikke kort, men heller lang.

But the original sentence is absolutely correct and natural.

Because lang and kort are adjectives used after the verb er. In that position, Norwegian does not use an article before them.

This is the same as in English:

• The sleeve length is long.
• not The sleeve length is a long.

So in Norwegian:

• Ermelengden er kort.
• Ermelengden er lang.

No article is needed.

The structure is:

• Ermelengden = subject
• er = verb
• ikke kort = negated predicate
• men tvert imot lang = contrasting predicate

So the sentence pattern is basically:

• [Subject] + [verb] + [not + adjective], but [on the contrary + adjective]

A very similar example would be:

• Boka er ikke kjedelig, men tvert imot spennende. = The book is not boring, but on the contrary exciting.