Í þann mund sem aðalskoðunin byrjar, stöðvast kílómetramælirinn.

Just as the periodic inspection begins, the odometer stops.

Breakdown of Í þann mund sem aðalskoðunin byrjar, stöðvast kílómetramælirinn.

• byrja: to begin
• stöðva: to stop
• aðalskoðunin: the periodic vehicle inspection
• kílómetramælirinn: the odometer

Questions & Answers about Í þann mund sem aðalskoðunin byrjar, stöðvast kílómetramælirinn.

Why does the sentence begin with Í þann mund sem? What kind of expression is that?

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.

Why is it þann mund and not some other form?

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.

What does sem do here?