Jo lavere månedsprisen er, desto lettere er det at vælge mobilselskabet.

It is the Danish way to express the idea the ... the ... in English.

So:

• Jo lavere månedsprisen er, desto lettere er det ...
• = The lower the monthly price is, the easier it is ...

This pattern shows a relationship between two changes:

• one thing changes
• and another thing changes as a result

A few similar examples:

• Jo mere du øver, desto bedre bliver du.
  = The more you practise, the better you become.

• Jo før, desto bedre.
  = The sooner, the better.

They are comparative forms of adjectives.

• lav = low

• lavere = lower

• let = easy / light
• lettere = easier / lighter

In a jo ... desto ... sentence, Danish normally uses the comparative, because the whole structure is about comparison:

• jo lavere ... = the lower ...
• desto lettere ... = the easier ...

So the sentence is not just describing the price as low and the choice as easy. It is saying:

• if the price becomes lower
• then choosing becomes easier

This is just how the comparative is formed from let.

• let → lettere
• lettest = easiest

The extra t is part of the normal spelling pattern here. Danish often changes spelling slightly when endings are added.

Also remember that let can mean both:

• easy
• light

In this sentence, lettere clearly means easier, not lighter.

Because Danish makes compound nouns very freely, much more than English does.

So:

• måned = month
• pris = price
• månedspris = monthly price

Then with the definite ending:

• månedsprisen = the monthly price

This is extremely normal in Danish. English often writes these as two words, but Danish usually joins them into one.

Other examples:

• mobiltelefon
• bilnøgle
• arbejdsdag

So månedsprisen is not unusual at all.

The ending -en is the definite article attached to the noun.

• en månedspris = a monthly price
• månedsprisen = the monthly price

Danish often puts the definite article at the end of the noun instead of using a separate word like English the.

Here, månedsprisen suggests a specific monthly price, probably the one connected to the mobile company or plan being discussed.

Because in this construction, the jo-part usually behaves like a subordinate clause, so the word order is not normal main-clause inversion.

Compare:

Normal main clause:

• Månedsprisen er lavere.

If you front an element in a normal main clause, Danish usually has verb-second word order:

• I dag er månedsprisen lavere.

But in the jo-clause, Danish normally keeps:

• jo lavere månedsprisen er

So the order is:

• jo lavere = fronted comparative phrase
• månedsprisen = subject
• er = verb

That is why månedsprisen er sounds right here.

Because the desto-clause usually behaves like a main clause, and Danish main clauses follow verb-second word order.

Here the fronted element is:

• desto lettere

After that, the finite verb comes second:

• er

Then the subject:

• det

So the structure is:

• desto lettere
  • er
    • det

This is completely normal Danish main-clause order.

Here det is a dummy subject or anticipatory subject.

The real idea is the infinitive phrase:

• at vælge mobilselskabet = to choose the mobile company

But Danish often uses det first and places the infinitive later:

• Det er let at vælge ...
• = It is easy to choose ...

So:

• desto lettere er det at vælge mobilselskabet
• literally: the easier is it to choose the mobile company

This works like English it in sentences such as:

• It is easy to understand.
• It is hard to choose.

The det does not refer to a thing; it is just filling the subject position.

Because this is not the ordinary kind of comparison.

Normally, Danish uses end for direct comparison:

• Prisen er lavere end før.
• = The price is lower than before.

But in the jo ... desto ... pattern, Danish does not use end.
Instead, the two comparative parts are linked to each other:

• jo lavere ..., desto lettere ...

So this is a different structure from X is lower than Y.

Often, yes. Danish very commonly uses both patterns:

• jo ..., desto ...
• jo ..., jo ...

For example:

• Jo lavere månedsprisen er, jo lettere er det at vælge mobilselskabet.

This means the same thing.

Very generally:

• jo ... desto ... can sound a bit more formal or written
• jo ... jo ... is very common and natural, especially in everyday speech

Both are useful to recognize.

Because after det er let/svært/nemt ..., Danish normally uses an infinitive with at.

So:

• Det er let at vælge.
• = It is easy to choose.

In your sentence:

• desto lettere er det at vælge mobilselskabet

The phrase at vælge mobilselskabet functions as the action that is easy.

This is very similar to English:

• It is easy to choose the company.

Mobilselskabet is the definite form:

• et mobilselskab = a mobile company / a mobile provider
• mobilselskabet = the mobile company / the mobile provider

Using the definite form suggests a specific company, or the company being discussed in context.

If the meaning were more general, Danish might also use:

• at vælge et mobilselskab
• = to choose a mobile company / provider

So the definite form here is probably context-driven. Without context, both could be possible, but they mean slightly different things.

Yes. The structure is very common, even if this exact sentence is a bit formal or written in style.

The important pattern is extremely useful:

• Jo mere ..., desto/jo mere ...
• Jo bedre ..., desto/jo bedre ...
• Jo længere ..., desto/jo længere ...

A learner will hear and read this kind of structure often, especially in:

• explanations
• advertisements
• formal writing
• advice
• general statements

So it is worth learning well.