Jo mørkere det blir, desto viktigere er hodelykten.

It expresses a linked comparison: as one thing changes, another changes too. It corresponds to English the ..., the ..., as in the darker it gets, the more important the headlamp is.

So in this sentence:

• jo mørkere det blir = as it gets darker
• desto viktigere er hodelykten = the more important the headlamp is

This pattern is very common in Norwegian.

Because they are comparative forms of adjectives.

• mørk = dark

• mørkere = darker

• viktig = important
• viktigere = more important

In Norwegian, many adjectives form the comparative by adding -ere, similar to English adjectives like dark → darker.

For many common adjectives, Norwegian prefers a built-in comparative form rather than using mer.

So:

• viktig → viktigere
• mørk → mørkere

Using mer viktig is generally not the normal choice here. Norwegian often uses mer with longer adjectives or in cases where a regular comparative is awkward, but with viktig, viktigere is the standard form.

Here det is a formal or anticipatory subject, similar to English it in it gets dark.

Norwegian often uses det with weather, time, light, and general situations:

• Det er kaldt. = It is cold.
• Det blir sent. = It is getting late.
• Det blir mørkt. = It is getting dark.

So det blir here means something like it becomes / it gets.

Because blir shows change, while er shows a state.

• det er mørkt = it is dark
• det blir mørkt = it is getting dark / it becomes dark

In this sentence, the idea is not just darkness as a fixed condition, but the process of it becoming darker. That is why blir fits well.

Because Norwegian is a verb-second language in main clauses. That means the finite verb usually comes in the second position.

Here, the sentence begins with desto viktigere, so the verb er must come next:

• Desto viktigere er hodelykten.

If you put hodelykten before er, the structure would not match normal main-clause word order.

This is a very common thing for English speakers to notice, because English word order is less strict in this particular way.

Hodelykten is the definite form: the headlamp.

• en hodelykt = a headlamp
• hodelykten = the headlamp

Norwegian often uses the definite form when talking about a specific thing that is understood in context, or when speaking generally about the relevant item in a situation. In a sentence like this, it sounds natural to refer to the headlamp as the important item.

It comes from:

• hode = head
• lykt = lamp / flashlight / torch
• hodelykt = headlamp / head torch
• hodelykten = the headlamp

So it is a compound noun, which is very common in Norwegian. Norwegian often joins words together into one noun instead of writing them separately.

Yes. You may hear both patterns:

• jo mørkere ..., desto viktigere ...
• jo mørkere ..., jo viktigere ...

Both are used. Jo ..., desto ... is very standard and clear, and jo ..., jo ... is also common in everyday Norwegian. A learner should recognize both.

Not old-fashioned, but it can sound a little more formal or written than repeating jo in both halves. Still, it is completely normal and modern Norwegian, and you will see it often in writing and hear it in speech as well.

So this sentence sounds natural.

No, this is a different use.

Norwegian jo can do more than one job:

• as a discourse particle: Det er jo sant. = It is, after all, true / It is obviously true.
• as part of the comparative pattern: jo ..., desto ...

In your sentence, jo is not adding emphasis in that conversational sense. It is part of a fixed comparison structure.

Yes. A more direct version could be something like:

• Når det blir mørkere, blir hodelykten viktigere.

That means roughly when it gets darker, the headlamp becomes more important.

This version is grammatically simpler for learners, but the original jo ..., desto ... version is more idiomatic when you want to stress the proportional relationship.

A useful breakdown is:

• Jo mørkere det blir
  comparative clause: as it gets darker

• desto viktigere er hodelykten
  main clause: the more important the headlamp is

So the sentence is built from two linked comparative parts. The first sets up the changing condition, and the second shows the result. Once you learn this pattern, you can use it with many other adjectives:

• Jo eldre man blir, desto mer erfaring får man.
• Jo mer du øver, desto bedre blir du.

It is a very productive structure in Norwegian.