Glasuren har et blått mønster.

Because mønster is a neuter noun in Norwegian.

That means:

• et mønster = a pattern
• not en mønster

When an adjective describes a neuter singular noun in the indefinite form, the adjective usually gets -tt:

• blå → blått

So:

• et blått mønster = a blue pattern

If the noun were a common-gender noun, you would usually get:

• en blå bil = a blue car

This is adjective agreement.

In Norwegian, adjectives change form depending on the noun they describe. For blå, the most important patterns are:

• en blå stol = a blue chair
• et blått mønster = a blue pattern
• blå stoler = blue chairs
• det blå mønsteret = the blue pattern

So blått is the form used with a singular neuter indefinite noun.

This is one of the very common adjective patterns in Norwegian, so it is worth memorizing early.

In Norwegian, the definite article is often attached to the end of the noun.

So:

• glasur = glaze / icing
• glasuren = the glaze / the icing

That -en ending is the definite ending for many common-gender nouns.

This is different from English, where the is a separate word.

Compare:

• en glasur = a glaze
• glasuren = the glaze

If you add an adjective, Norwegian often uses both a separate definite word and the noun ending:

• den blå glasuren = the blue glaze

This is often called double definiteness.

Har is the present tense of å ha, meaning to have.

So the sentence literally says that the glaze has a blue pattern.

Norwegian often uses har the same way English does when describing features or characteristics of something:

• Huset har en stor hage = The house has a big garden
• Koppen har et rødt merke = The cup has a red mark

If you used er, the structure would be different. For example:

• Mønsteret er blått = The pattern is blue

So:

• har et blått mønster = has a blue pattern
• er blått = is blue

Both are possible in Norwegian, but they mean slightly different things.

In this sentence:

• glasur is a common-gender noun

  • en glasur
  • glasuren

• mønster is a neuter noun

  • et mønster
  • mønsteret

This matters because Norwegian grammar depends a lot on noun gender. The gender affects:

• the article (en / et)
• the adjective form (blå / blått)
• the definite ending (-en / -et)

That is why the sentence contains:

• glasuren
• et blått mønster

Because that is the normal position for an adjective directly describing a noun in Norwegian.

So:

• et blått mønster = a blue pattern
• en stor bil = a big car
• et gammelt hus = an old house

This works much like English.

If the adjective comes after the noun, the sentence usually needs a verb:

• Mønsteret er blått = The pattern is blue

So in your sentence, blått comes before mønster because it is part of the noun phrase et blått mønster.

Here are some useful related forms:

Indefinite singular

• et blått mønster = a blue pattern

Definite singular

• det blå mønsteret = the blue pattern

Indefinite plural

• blå mønstre = blue patterns

Definite plural

• de blå mønstrene = the blue patterns

A helpful thing to notice is that blått only appears in the neuter singular indefinite form. In the plural and definite forms, you normally get blå.

So you can think of it like this:

• et blått mønster
• det blå mønsteret
• blå mønstre
• de blå mønstrene

Yes. That is also correct, but it is a slightly different way of presenting the information.

Compare:

• Glasuren har et blått mønster.
  = The glaze has a blue pattern.

• Det er et blått mønster på glasuren.
  = There is a blue pattern on the glaze.

The first sentence focuses on the glaze as the subject.
The second sentence introduces the pattern with there is-style wording.

Both are natural. Norwegian often allows both kinds of structure, just like English does.

The two vowels many English speakers notice here are:

• å in blått
• ø in mønster

A rough guide:

• å is often similar to the vowel in English saw or law
• ø has no exact English equivalent, but it is somewhat similar to the vowel in British English bird, just without the r

So very roughly:

• blått sounds a bit like blawt
• mønster starts with a vowel that English speakers often need practice with

Also note:

• tt in blått is written double, but the main thing for learners is usually getting the vowel right
• mønster is a neuter noun, and the ø is one of the classic Norwegian vowel sounds to train carefully

If you want to sound natural, the vowel quality matters more than trying to force an English-style pronunciation.