Bíllinn kostar tuttugu þúsund.

In Icelandic, the definite article is usually attached to the noun.

• bíll = car
• bíllinn = the car

So -inn here is the masculine singular nominative form of the definite article. Icelandic does have separate words that can function a bit like the in some contexts, but the normal everyday pattern is the suffix article.

This sentence also shows a useful contrast with English: Icelandic normally has no true indefinite article. So bíll can mean a car or just car, depending on context.

The infinitive is að kosta = to cost.

In the present tense, the form changes depending on the subject:

• ég kosta = I cost
• þú kostar = you cost
• hann/hún/það kostar = he/she/it costs

Since bíllinn is singular and means the car / it, the correct form is kostar.

So:

• Bíllinn kostar ... = The car costs ...

Icelandic often leaves the currency unstated if it is clear from context.

So Bíllinn kostar tuttugu þúsund can mean The car costs twenty thousand, with the currency understood.

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

• Bíllinn kostar tuttugu þúsund krónur.

In another context, you could also say things like:

• ... tuttugu þúsund evrur
• ... tuttugu þúsund dollara

Because this is an exact number expression: twenty thousand.

In Icelandic, exact numbers normally use þúsund:

• eitt þúsund = one thousand
• tvö þúsund = two thousand
• tuttugu þúsund = twenty thousand

The plural þúsundir usually means thousands in a looser, more general sense:

• Þúsundir manna komu. = Thousands of people came.

So:

• tuttugu þúsund = exact number
• þúsundir = thousands, in general

The verb að kosta normally takes the amount/price as an accusative object.

In this sentence, that is not very visible on the surface, because:

• tuttugu does not change here
• þúsund in this kind of number phrase also does not clearly show case
• if you add krónur, that form is the same in nominative and accusative plural

So even though the case is grammatically important, the form in this sentence looks the same.

A fuller version is:

• Bíllinn kostar tuttugu þúsund krónur.

It is built very much like English:

• tuttugu = twenty
• þúsund = thousand

So tuttugu þúsund is literally twenty thousand.

If you add a noun after it, that noun comes after the whole number phrase:

• tuttugu þúsund krónur = twenty thousand kronor

The letter þ is pronounced like th in thing, not like th in this.

So þúsund begins with a voiceless th sound.

A rough English-friendly approximation is:

• þúsund ≈ THOO-sund

That is only approximate, but it is a good start.

The ll in Icelandic is often not pronounced like ordinary English l+l.

In words like bíllinn, many speakers use a sound that learners often approximate as something like tl. So a rough learner-friendly approximation is:

• bíllinn ≈ BEE-tlin

That is not exact, but it points you in the right direction.

Also, Icelandic words are usually stressed on the first syllable, so the stress is on:

• BÍL-linn
• KOS-tar
• TUT-tugu
• ÞÚ-sund

The accents in Icelandic are part of the spelling of the vowel. They are not just optional stress marks.

For example:

• í is a different vowel from i
• ú is a different vowel from u

A rough English approximation is:

• í ≈ ee in see
• ú ≈ oo in food

Also, Icelandic stress normally falls on the first syllable anyway, so the accent marks are not mainly there to show stress.

The neutral, most natural order here is:

• Bíllinn kostar tuttugu þúsund.

That is subject + verb + amount.

Icelandic does allow other word orders for emphasis, but because it is a verb-second language, if you move something to the front, the finite verb usually stays in second position:

• Tuttugu þúsund kostar bíllinn.

That is possible, but it sounds more marked or emphatic than the normal version. For a basic statement, the original sentence is the best model to learn first.