Urglasset er ikke mørkt; tvert imot er det klart.

Because urglasset is the definite singular form of the noun.

• et urglass = an hourglass / a glass
• urglasset = the hourglass / the glass

Norwegian usually adds the definite article to the end of the noun instead of putting a separate word in front like English the.

Since urglass is a neuter noun, the definite ending is -et.

Because the noun being described is urglasset, and that noun is neuter singular.

In Norwegian, adjectives often agree with the noun:

• common gender singular: mørk, klar
• neuter singular: mørkt, klart
• plural / definite: often mørke, klare

Here the adjectives come after er, so they are predicate adjectives, but they still agree with the subject:

• Urglasset er mørkt
• Urglasset er klart

That is the normal word order in a main clause in Norwegian.

In a main clause, the finite verb usually comes early, and ikke usually comes after it:

• Urglasset er ikke mørkt

Compare a subordinate clause, where ikke usually comes before the verb:

• ... fordi urglasset ikke er mørkt

So the placement of ikke is a very common Norwegian word-order pattern.

Tvert imot is a fixed expression meaning on the contrary, quite the opposite, or rather the opposite.

It is used when the second statement strongly corrects or reverses the first one.

So:

• Urglasset er ikke mørkt; tvert imot er det klart.

means something like:

• The hourglass is not dark; on the contrary, it is clear.

It is stronger than just but.

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

Here, tvert imot takes the first position. That means the verb er must come next:

• Tvert imot
  • er
    • det
      • klart

So tvert imot det er klart is not correct standard word order in a main clause.

Because det refers back to urglasset.

Since urglasset is a neuter noun, the pronoun used for it is det.

This is very natural in Norwegian, just as English often says it instead of repeating the noun:

• Urglasset er ikke mørkt; tvert imot er det klart.
• The hourglass is not dark; on the contrary, it is clear.

Repeating urglasset would also be possible, but it would sound more repetitive:

• ... tvert imot er urglasset klart

Here it really refers to urglasset.

So this is not the same as the dummy det in sentences like:

• Det regner = It is raining
• Det er kaldt = It is cold

In your sentence, det stands for a specific thing already mentioned: urglasset.

The semicolon links two closely connected independent clauses:

• Urglasset er ikke mørkt
• tvert imot er det klart

A semicolon is stronger than a comma and weaker than a full stop. It works well here because the second clause directly contrasts with the first.

You could also write:

• Urglasset er ikke mørkt. Tvert imot er det klart.

That would also be correct.

Here it is an adjective.

It describes the subject (det, referring to urglasset) after the verb er:

• det er klart

So klart means that the thing itself is clear.

Norwegian klart can also be an adverb in other sentences, but not here. In this sentence it is definitely an adjective in the neuter singular form.

Yes. A simpler version could be:

• Urglasset er ikke mørkt, men klart.

That means roughly the same thing.

However, tvert imot adds a stronger sense of contradiction, something like not dark — quite the opposite, clear. So the original sentence sounds a bit more emphatic or formal.

Not always in every context.

• mørk / mørkt usually means dark
• klar / klart can mean clear, bright, distinct, or ready, depending on context

So they are not perfect dictionary opposites in every situation. But in this sentence they are being used as a strong contrast: not dark, but clear/bright.

That is why tvert imot fits well here.