Sømmonnet er ikke bredt. Tvert imot er det smalt.

Because Norwegian usually writes compound nouns as a single word.

Here, sømmonn is a compound:

• søm = seam
• monn = allowance, margin

So sømmonn is one noun, not two separate words. This is very normal in Norwegian. English often uses separate words where Norwegian prefers one compound word.

The ending -et marks the definite singular of a neuter noun.

The dictionary form is:

• et sømmonn = an indefinite singular form

The sentence uses:

• sømmonnet = the definite singular form

So the pattern is:

• et sømmonn = a seam allowance
• sømmonnet = the seam allowance

This is a very common neuter pattern in Norwegian:

• et hus → huset
• et barn → barnet
• et sømmonn → sømmonnet

Because they are agreeing with a neuter singular noun.

The noun sømmonn is neuter:

• et sømmonn

When an adjective describes a neuter singular noun, it often takes -t:

• bred → bredt
• smal → smalt

That is true even after er, where the adjective is used predicatively:

• Sømmonnet er bredt
• Sømmonnet er smalt

A useful comparison:

• en vei er bred
• et rom er bredt

So bredt and smalt are there because the thing being described is a neuter noun.

Because det is the pronoun used for a neuter singular noun.

Since sømmonnet is neuter, Norwegian can replace it with det:

• Sømmonnet er ikke bredt.
• Tvert imot er det smalt.

Here, det means sømmonnet. It avoids repeating the full noun again.

If the noun were common gender, you would normally use den instead:

• Boka er ny. Den er interessant.

But with a neuter noun:

• Huset er stort. Det er gammelt.

Yes, tvert imot is a fixed expression. It means something like on the contrary or quite the opposite.

It is used to strongly reverse what came before.

So the structure here is:

• first: a statement is denied
• then: tvert imot introduces the opposite idea

It is a very natural expression in Norwegian and worth learning as a whole chunk.

Because Norwegian main clauses follow the V2 rule: the finite verb must come in the second position.

In a neutral sentence, you would have:

• Det er smalt

But when tvert imot is moved to the front, it takes the first position. Then the finite verb er must come next:

• Tvert imot er det smalt

So the order is:

1. Tvert imot
2. er
3. det
4. smalt

This inversion is very common in Norwegian whenever something other than the subject comes first.

Compare:

• Det er smalt
• I dag er det smalt
• Tvert imot er det smalt

In a normal Norwegian main clause, ikke usually comes after the finite verb.

So:

• Sømmonnet er ikke bredt

This is the standard placement in main clauses.

A very useful contrast is subordinate clauses, where ikke usually comes before the verb:

• Jeg vet at sømmonnet ikke er bredt

So:

• main clause: er ikke
• subordinate clause: ikke er

This is one of the most important word-order differences for English speakers to learn.

Yes. You could say:

• Tvert imot er sømmonnet smalt.

That is grammatical. But using det sounds more natural in many contexts because it avoids repetition.

So both are possible, but:

• ... er det smalt = more natural, smoother
• ... er sømmonnet smalt = more explicit, slightly heavier

Yes.

The basic adjective is smal. Its neuter singular form is smalt.

A simple pattern is:

• en smal vei
• et smalt rom

And after er:

• Veien er smal
• Rommet er smalt

The same kind of change happens with bred:

• en bred vei
• et bredt rom

So in your sentence, smalt is not a different word with a different meaning. It is just the form that matches a neuter singular noun.

Yes. The sentence is making a stronger point than simply saying one thing is not true.

• ikke bredt only says that it is not wide
• smalt says positively that it is narrow

Then tvert imot strengthens the contrast even more. So the overall effect is:

• not just not wide
• but actually narrow

That makes the sentence sound clear and emphatic.