Bíllinn stenst aðalskoðunina.

Because -inn is the definite article attached to the noun. Icelandic often puts the at the end of the word instead of using a separate word before it.

• bíll = car
• bíllinn = the car

So bíllinn is literally car-the.

In ordinary Icelandic, the definite article is usually a suffix, not a separate word.

So instead of:

• the car
• the inspection

you get:

• bíllinn
• aðalskoðunina

Icelandic can use separate words like hinn, but that is not the normal choice in a simple sentence like this.

Stenst comes from the infinitive standast.

In this sentence, stenst is the 3rd person singular present form, agreeing with bíllinn.

So:

• standast = to pass, withstand, meet the requirements of
• stenst = passes / withstands / meets

Here the meaning is passes.

Because it is an irregular verb. Icelandic strong verbs often change their vowel when they are conjugated.

So the jump from standast to stenst is something you mostly have to learn as part of the verb’s pattern. This is very common in Icelandic and not unusual.

A good practical approach is to learn:

• standast = infinitive
• stenst = present singular form you will often see

Not in a simple word-for-word way here. For learners, the safest approach is to treat standast as a whole dictionary verb.

Many Icelandic verbs ending in -st are old middle-voice forms, but in modern Icelandic they are often best learned as complete verbs with their own meanings. So here, don’t try to split stenst into separate meaningful pieces in the same way you might in English. Just learn:

• standast = to pass / withstand
• stenst = passes

It is a compound noun plus the definite ending.

You can break it down like this:

• aðal- = main, principal, primary
• skoðun = inspection, examination
• -ina = a feminine singular definite ending here

So aðalskoðun literally means main inspection. In context, it usually refers to the official vehicle inspection.

Because this noun is the direct object of the verb stenst, and here it is in the accusative case.

For this feminine noun:

• aðalskoðunin = the main inspection in the nominative
• aðalskoðunina = the main inspection in the accusative

Since standast takes a direct object, Icelandic uses the accusative form here.

Bíllinn is in the nominative case because it is the subject of the sentence — it is the thing doing the passing.

So the basic structure is:

• bíllinn = subject, nominative
• stenst = verb
• aðalskoðunina = object, accusative

A useful contrast is:

• bíllinn = the car (nominative)
• bílinn = the car (accusative)

That double ll vs single l difference is something learners often need to watch carefully.

Yes. This is the most neutral, straightforward order:

• Subject + Verb + Object

So:

• Bíllinn = subject
• stenst = verb
• aðalskoðunina = object

That is a completely natural way to say it.

Icelandic does allow other word orders for emphasis, but this version is the plain, unmarked one.

Yes. Standast is used quite broadly for the idea of passing, meeting a test, or holding up under something.

For example, it can be used with:

• exams
• tests
• requirements
• scrutiny
• pressure

So the core idea is not just to pass a vehicle inspection, but more generally to pass / withstand / meet the standard of something.

In everyday Icelandic, especially with cars, it usually refers to the official required inspection, not just any inspection somebody happens to call important.

So although the word is built literally from main + inspection, in practice it often means something like:

• the official vehicle inspection
• the main mandatory inspection

That is why this sentence sounds natural in a vehicle-testing context.