Han kjøpte en ny dress.

Because dress is a common-gender noun in Norwegian, so it takes en in the singular indefinite form.

A few related forms:

• en dress = a suit
• dressen = the suit

In Bokmål, nouns are usually either:

• common gender → takes en
• neuter → takes et

So you have to learn the gender together with the noun: en dress.

The adjective changes form to match the noun.

Here, dress is:

• singular
• indefinite
• common gender

So the correct adjective form is ny.

Compare:

• en ny dress = a new suit
• et nytt hus = a new house
• nye sko = new shoes
• den nye dressen = the new suit

So ny is the form used with a singular indefinite common-gender noun.

Kjøpte is the simple past (also called the preterite).

The verb is å kjøpe = to buy.

Important forms:

• å kjøpe = to buy
• kjøper = buys / is buying
• kjøpte = bought
• har kjøpt = has bought

So Han kjøpte en ny dress uses the past tense to describe a completed action in the past.

Because å is used with the infinitive form of a verb, not with a finite past-tense verb.

Compare:

• Han vil kjøpe en ny dress. = He wants to buy a new suit.
• Han kjøpte en ny dress. = He bought a new suit.

In the sentence you gave, kjøpte is already the main verb in the past tense, so å is not used.

No — this is a very important false friend.

In Norwegian, dress usually means suit.

So:

• en dress = a suit
• en kjole = a dress

That means an English speaker can easily misunderstand this word at first.

In this sentence, the word order is very similar to English:

• Han = subject
• kjøpte = verb
• en ny dress = object

So it follows a basic subject–verb–object pattern.

However, Norwegian main clauses also follow the V2 rule, which means the finite verb normally comes in the second position.

For example:

• Han kjøpte en ny dress i går.
• I går kjøpte han en ny dress.

In both cases, the verb kjøpte stays in the second position.

Usually, no.

With a singular countable noun like dress, Norwegian normally needs an article in this kind of sentence:

• Han kjøpte en ny dress.

Leaving out en would sound incomplete in standard Norwegian in most normal contexts.

You may sometimes see article omission in:

• headlines
• notes
• some fixed expressions
• certain dialectal or stylistic uses

But for a learner, en ny dress is the correct standard form.

You would say den nye dressen.

This is because Norwegian often uses double definiteness:

• a definite article-like word before the noun: den
• the noun itself also gets a definite ending: -en

So:

• en ny dress = a new suit
• den nye dressen = the new suit

Notice that the adjective also changes:

• ny → nye

A rough guide:

• kj is a soft sound that does not exist exactly in most English accents. It is similar to the sound in German ich for many speakers.
• ø is a rounded vowel, somewhat like the vowel in British English bird, but with rounded lips.
• the final e is usually weak, like uh.

So kjøpte is roughly something like KHYURP-tuh or HYURP-tuh, but no English spelling is perfect.

The trickiest part for English speakers is usually ø and kj.

It is capitalized only because it is the first word of the sentence.

In Norwegian, pronouns like han, hun, jeg, and du are not normally capitalized in the middle of a sentence.

So:

• Han kjøpte en ny dress.
• Jeg så at han kjøpte en ny dress.

In the second sentence, han is lowercase because it is not sentence-initial.