Learn Hardcore Norwegian: Koppen er varm, men regnet og skyene gjør morgenen kjøligere. - The cup is warm, but the rain and the clouds make the morning cooler.

-en is the definite singular ending for common-gender nouns (en-words), e.g. kopp → koppen (the cup).
-et is the definite singular ending for neuter nouns (et-words), e.g. regn → regnet (the rain).
-ene is the definite plural ending, e.g. skyer → skyene (the clouds).
Likewise morgenen is “the morning” (en-word, definite singular).

Why is the adjective varm not varmt in Koppen er varm?

Adjectives in predicate position agree with the gender of the noun they describe. Kopp is a common-gender noun (en-word), so you use the common-gender form varm. If you described a neuter noun (et-word), you would say varmt, e.g. Huset er varmt (The house is warm).

How is kjøligere formed, and what does it mean?

Kjøligere is the comparative form of kjølig (cool). You form it by adding -ere to the adjective stem:
kjølig → kjøligere
It means “cooler” (i.e. more cool).

Could you say mer kjølig instead of kjøligere?

Yes, you can technically use mer kjølig (“more cool”), especially for emphasis or in more formal styles. However, the single-word comparative kjøligere is far more common in everyday speech and writing.

What does the verb gjør mean here, and how is it used?

Here gjør is the present-tense form of gjøre, which in this context means “make” or “cause”. The structure is:
Subject (regnet og skyene) + gjør + object (morgenen) + complement (kjøligere)
Literally: “The rain and the clouds make the morning cooler.”

Why does the sentence use men between the two clauses and og between regnet and skyene?

men means “but” and introduces a contrast between the first clause (Koppen er varm) and the second clause (regnet og skyene gjør … kjøligere).
og means “and” and simply links two agents in the second clause: the rain and the clouds.

Why is there a comma before men?

In Norwegian, when you join two independent clauses with a coordinating conjunction like men, you normally place a comma before it. It signals that what follows is a new clause with its own subject and verb.

Why does the word order in the second clause go regnet og skyene gjør instead of gjør regnet og skyene?

Norwegian main clauses follow the V2 rule (“verb second”): the finite (conjugated) verb must be in the second position. After the conjunction men, the subject regnet og skyene comes first, and the verb gjør comes second. If you started with an adverb or object, you’d still keep the verb in second position.