Det koster én milliard.

Det means it here.

Just like English often says It costs..., Norwegian usually needs a subject too. In this sentence, det can refer to some thing already known from context, or it can function as a general it.

For example:

• Huset koster én milliard. = The house costs one billion.
• Det koster én milliard. = It costs one billion.

So det is completely normal here.

Koster is the present tense of the verb å koste, which means to cost.

So:

• å koste = to cost
• koster = costs / is costing

In this sentence, Det koster én milliard means that something costs one billion.

Én is the numeral one, written with an accent to make it clearly mean the number.

In many cases, Norwegian can also write en milliard. The accent in én is often used when the writer wants to emphasize that it is specifically one and not some other number.

So these are both possible:

• Det koster en milliard.
• Det koster én milliard.

The version with én makes the number more explicit.

Because milliard is a common gender noun in Norwegian, not a neuter noun.

Norwegian one changes form depending on gender when used directly with a noun:

• en / én for common gender nouns
• et / ett for neuter nouns

Since milliard takes en, you get:

• én milliard

You would use ett with a neuter noun, for example:

• ett hus = one house

Not in normal modern English usage.

For an English speaker, the important point is:

• Norwegian milliard = English billion = 1,000,000,000 = 10⁹

This is a very important vocabulary trap, because Norwegian also has the word billion, but in Norwegian that means 10¹², not English billion.

So:

• én milliard = one billion in English
• Norwegian én billion = English one trillion

Because Norwegian often leaves the currency unstated when the context already makes it clear.

So Det koster én milliard simply means It costs one billion, and the currency is understood from the situation.

If you want to state the currency explicitly, you can add it:

• Det koster én milliard kroner. = It costs one billion kroner.

Yes. This is a normal Norwegian main clause:

• Det = subject
• koster = verb
• én milliard = amount/object-like complement

So the structure is basically the same as English:

• It costs one billion.

If you turn it into a yes/no question, Norwegian usually puts the verb first:

• Koster det én milliard? = Does it cost one billion?

Yes. If you want to name what costs that much, you can replace det with the noun phrase.

For example:

• Bilen koster én milliard. = The car costs one billion.
• Prosjektet koster én milliard. = The project costs one billion.

So det is only used because the thing is not named directly in this sentence.

A careful approximate pronunciation is:

de KOS-ter ayn mil-YARD

A few useful notes:

• det is very often pronounced more like de in everyday speech
• koster has stress on the first syllable
• milliard has its strongest stress near the end

Exact pronunciation varies by dialect, but if you say something close to de KOS-ter ayn mil-YARD, you will be understood.

Then milliard becomes plural: milliarder.

For example:

• én milliard = one billion
• to milliarder = two billion
• fem milliarder = five billion

So in your sentence, milliard is singular because the amount is one.