Í þann mund sem hann athugar handbremsuna, slokknar viðvörunarljósið.

It means just as, right at the moment when, or at the very moment that.

It is best learned as a fixed expression. In this sentence:

Í þann mund sem hann athugar handbremsuna, slokknar viðvörunarljósið.

the first part sets up the timing: just as he checks the handbrake.

That is a very natural question. Normally, yes, í can take either:

• dative for location: í bílnum = in the car
• accusative for motion into something: í bílinn = into the car

But í þann mund is an idiomatic expression. It uses the accusative as part of a fixed phrase, and learners usually just memorize the whole chunk:

í þann mund = at that moment / just then

So even if it does not fit the most basic location-vs-motion rule in an obvious way, this expression is standard and should be treated as a set phrase.

In this phrase, mund means something like moment or point in time.

You will mostly meet it in the expression í þann mund rather than as a very common everyday standalone noun. So for practical learning, it is more useful to recognize the whole phrase than to think of mund as a word you will constantly use by itself.

Here sem means when or as.

So:

• Í þann mund sem hann athugar handbremsuna
  = just as he checks the handbrake

In other contexts, sem can also mean that, which, or introduce relative clauses, so it is a very flexible word. In this sentence, though, it introduces a time clause.

This is because of Icelandic V2 word order (verb-second word order).

The sentence begins with a fronted time expression plus subordinate clause:

Í þann mund sem hann athugar handbremsuna

After that, the finite verb of the main clause comes next:

slokknar

And then the subject:

viðvörunarljósið

So the structure is:

• [Time clause]
• [finite verb]
• [subject]

That is very normal in Icelandic.

If you put the main clause in a more neutral order, it would be:

Viðvörunarljósið slokknar í þann mund sem hann athugar handbremsuna.

Both are grammatical, but the original sentence follows the common Icelandic pattern where the verb comes second after a fronted element.

Athugar is from the verb athuga, which means to check, to examine, to look into, or to inspect.

Here it means:

hann athugar handbremsuna = he checks the handbrake

It is a very useful everyday verb in Icelandic.

Because handbremsuna is:

• the definite form: the handbrake
• in the accusative singular, because it is the direct object of athugar

The base noun is:

• handbremsa = handbrake

With the suffixed definite article:

• handbremsan = the handbrake (nominative)
• handbremsuna = the handbrake (accusative)

So in this sentence:

• hann = subject
• athugar = verb
• handbremsuna = direct object

That is why the accusative form appears.

Because Icelandic forms compound nouns very freely, much like German.

viðvörunarljósið can be broken down as:

• viðvörun = warning
• linking form -ar-
• ljós = light
• -ið = the definite article

So the whole word means:

the warning light

This kind of long compound is completely normal in Icelandic.

Slokknar comes from slokkna, which means to go out, to be extinguished, or to switch off when talking about a light, flame, lamp, and similar things.

So:

viðvörunarljósið slokknar = the warning light goes out

A very important contrast is:

• slokkna = go out / become extinguished by itself or as the subject’s change of state
• slökkva = switch off / extinguish something

So:

• Ljósið slokknar. = The light goes out.
• Hann slökkvir ljósið. = He switches off the light.

That difference is very useful to learn early.

Both verbs are in the present tense:

• athugar = checks
• slokknar = goes out

This can simply describe an event in the present, but Icelandic also often uses the present in summaries, instructions, and vivid narration in ways that may feel slightly broader than basic English present-tense use.

So the sentence can sound like a straightforward description of what happens:

Just as he checks the handbrake, the warning light goes out.

Yes. Here is a case breakdown:

• hann — nominative
  Subject of athugar

• handbremsuna — accusative singular
  Direct object of athugar

• viðvörunarljósið — nominative singular
  Subject of slokknar

• þann mund — accusative in a fixed idiomatic expression after í

So the sentence contains both main-clause and subordinate-clause grammar, and the cases are doing normal Icelandic jobs: subjects in the nominative, direct object in the accusative, plus one fossilized idiomatic phrase.

Yes. A learner might also encounter simpler alternatives such as:

• Þegar hann athugar handbremsuna, slokknar viðvörunarljósið.
• Um leið og hann athugar handbremsuna, slokknar viðvörunarljósið.

These are a bit more transparent for learners:

• þegar = when
• um leið og = as / just as / at the same time as

But í þann mund sem is perfectly good Icelandic and gives a slightly more literary or vivid sense of just at that moment.

Yes, the comma separates the initial subordinate time clause from the main clause:

• Í þann mund sem hann athugar handbremsuna,
• slokknar viðvörunarljósið.

That punctuation helps show the sentence structure clearly. It is especially useful when a long clause comes first.