Í þann mund sem aðalskoðunin byrjar, stöðvast kílómetramælirinn.

Í þann mund sem is a fixed time expression meaning something like just as, at the moment when, or right when.

It is idiomatic, so learners usually understand it best as a whole chunk rather than word-by-word at first. In this sentence, it introduces a time clause:

• Í þann mund sem aðalskoðunin byrjar
  = just as the main inspection begins

You may also see í þann mund að in similar contexts.

The phrase í þann mund is a fixed idiom, and þann is the masculine accusative singular form of sá (that).

Normally, í often takes the dative when it means location, but with many time expressions and set phrases, Icelandic uses forms that have to be learned as idioms. So here, it is best to memorize:

• í þann mund = at that moment / just then

The noun mundur is not very common in everyday speech outside set expressions like this one, so most learners simply learn the whole phrase.

Here sem introduces the subordinate clause, much like when or that in English, depending on the structure.

So:

• Í þann mund sem aðalskoðunin byrjar
• literally: at that moment that the main inspection begins
• naturally: just as the main inspection begins

In Icelandic, sem is very common in relative clauses and in certain fixed time expressions like this one.

The -in is the suffixed definite article, meaning the.

So:

• aðalskoðun = main inspection
• aðalskoðunin = the main inspection

Icelandic usually attaches the to the end of the noun instead of using a separate word like English does.

This noun is also feminine singular nominative here, because it is the subject of byrjar.

It is a compound noun:

• aðal- = main, principal
• skoðun = inspection, examination
• -in = the

So:

• aðalskoðun = main inspection
• aðalskoðunin = the main inspection

Compound words are extremely common in Icelandic, so it is useful to get used to breaking them into parts.

Byrjar is the 3rd person singular present tense of að byrja (to begin).

It matches the subject aðalskoðunin because that noun is singular:

• aðalskoðunin byrjar = the main inspection begins

The present tense is often used in Icelandic the same way it is in English for general statements, scheduled events, or narration.

This is because Icelandic is a verb-second language in main clauses.

When a sentence begins with something other than the subject—here, a time clause:

• Í þann mund sem aðalskoðunin byrjar

the finite verb of the main clause usually comes next:

• stöðvast

and then the subject follows:

• kílómetramælirinn

So the pattern is:

• subordinate clause first
• then finite verb
• then subject

That is why you get:

• Í þann mund sem aðalskoðunin byrjar, stöðvast kílómetramælirinn.

rather than a subject-first order in the main clause.

Yes. Stöðvast is the -st form of stöðva.

• stöðva usually means to stop something
• stöðvast usually means to stop, come to a stop, or be stopped

So:

• Ég stöðva bílinn = I stop the car
• Bíllinn stöðvast = The car stops

In your sentence:

• kílómetramælirinn stöðvast = the odometer stops

The -st ending often gives a middle/passive-like meaning, though its exact use depends on the verb.

It is nominative singular masculine, because it is the subject of the verb stöðvast.

Breakdown:

• kílómetramælir = odometer
• kílómetramælirinn = the odometer

The ending -inn is the suffixed definite article for a masculine singular noun in the nominative.

In this sentence, the odometer is the thing doing the stopping, so nominative makes sense.

It is another compound noun:

• kílómetra- = kilometre-
• mælir = meter, gauge
• -inn = the

So:

• kílómetramælir = odometer
• kílómetramælirinn = the odometer

Icelandic often uses transparent compounds like this, so recognizing the parts can make long words much easier to understand.

The comma separates the initial subordinate time clause from the main clause:

• Í þann mund sem aðalskoðunin byrjar,
• stöðvast kílómetramælirinn.

This is very normal punctuation in Icelandic when a subordinate clause comes first.

Yes. Icelandic often uses the present tense for actions that happen as part of a sequence, a routine, or a described process.

So even if the sentence refers to a specific situation, the present tense can still sound natural, especially in explanations, instructions, or formal descriptions.

Yes, but the emphasis and flow would change.

For example, you could have:

• Kílómetramælirinn stöðvast í þann mund sem aðalskoðunin byrjar.

That is also grammatical. It starts with the subject instead of the time clause.

The original version puts more focus on the timing:

• Just as the main inspection begins, ...

while the rearranged version begins with the thing affected:

• The odometer stops ...

Both are possible, but the original is very natural Icelandic.