Jo koldere det er, jo mere uld vil jeg have på.

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Breakdown of Jo koldere det er, jo mere uld vil jeg have på.

• være: to be
• det: it
• ville: to want

Questions & Answers about Jo koldere det er, jo mere uld vil jeg have på.

What does jo ... jo ... mean in this sentence?

It is the Danish pattern for the ... the ... in English.

So:

• Jo koldere det er = The colder it is
• jo mere uld vil jeg have på = the more wool I will want to have on / wear

This pattern is very common in Danish for showing that one thing changes together with another.

Examples:

• Jo mere jeg læser, jo mere lærer jeg.
  The more I read, the more I learn.
• Jo ældre han bliver, jo roligere bliver han.
  The older he gets, the calmer he becomes.

You may also see desto instead of the second jo, especially in slightly more formal Danish.

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Why is it koldere and not mere kold?

Because kold is a short adjective, and in Danish short adjectives usually make the comparative with -ere.

So:

• kold = cold
• koldere = colder
• koldest = coldest

Using mere with kold would sound wrong in standard Danish.

A quick comparison:

• kold → koldere
• stor → større
• lille → mindre
• interessant → mere interessant
  Here mere is used because the adjective is longer.

So in your sentence, koldere is the normal comparative form.

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Why is it mere uld and not flere uld?