Learn Hardcore Swedish: Hennes fönster är stort, men sovrummet är kallt på kvällen. - Her window is big, but the bedroom is cold in the evening.

Common gender singular (en-word): stor, kall (e.g., Bilen är stor)
Neuter singular (ett-word): stort, kallt (e.g., Huset är stort)
Plural: stora, kalla (e.g., Husen är stora) Predicative adjectives do not mark definiteness. You say Fönstret är stort (not stora).

How do I know fönster is singular here when it looks like a plural?

The adjective stort shows it’s neuter singular. The noun fönster has the same form in indefinite singular and indefinite plural. Forms:

ett fönster (indef. sg.)
fönstret (def. sg.)
flera fönster (indef. pl.)
fönstren (def. pl.) If it were plural here, the adjective would be plural: Hennes fönster är stora.

Why is it hennes fönster, not hennes fönstret?

A possessive determiner (min, din, hans, hennes, etc.) normally replaces the definite ending on the noun. So you say hennes fönster, not hennes fönstret. If you add an adjective, the adjective takes the definite form while the noun still has no definite ending: hennes stora fönster. Compare with double definiteness without a possessive: det stora fönstret.

Could I use sitt instead of hennes?

Use the reflexive possessive sin/sitt/sina only when the possessor is the grammatical subject of the same clause. Here the subject is Hennes fönster, so there’s no antecedent subject for sitt.

Main + subordinate (reflexive works): Hon tycker att sitt fönster är stort. (= her own)
Main + subordinate (non-reflexive → someone else’s): Hon tycker att hennes fönster är stort. You cannot say Sitt fönster är stort without a prior subject; it’s ungrammatical/odd.

Why is it sovrummet (“the bedroom”) instead of hennes sovrum (“her bedroom”)?

Swedish often uses a simple definite noun to refer to something contextually obvious without repeating the possessor. After mentioning something of hers, sovrummet naturally refers to the relevant bedroom in that context (likely hers). If you want to highlight ownership, you can say Hennes sovrum är kallt på kvällen. Both are fine, just a difference in focus.

What exactly does på kvällen mean? Can I say i kväll or på kvällarna?

på kvällen = in the evening (time of day; often general/habitual or on a day already understood from context)
i kväll = this evening/tonight (specific, “tonight”)
på kvällarna / om kvällarna = in the evenings (habitual) i kvällen is ungrammatical.

Can I move på kvällen to the front?

Yes. Swedish main clauses are V2 (the finite verb is in second position). If you front the time phrase, invert subject and verb: På kvällen är sovrummet kallt.

Do I need the comma before men?

Yes. When men connects two independent main clauses, standard Swedish uses a comma before it: …, men …. If the second part isn’t a full clause, you normally don’t use a comma.

Could I drop the second är and say … men sovrummet kallt på kvällen?

Not in normal prose. Omitting the copula is a stylistic, headline/poetic option. Keep är in regular sentences.

What’s the difference between men and utan?

Use men for a simple contrast (“but”). Use utan only after a negation to mean “but rather/instead”: Sovrummet är inte varmt, utan kallt.

Would blir be better if the room “gets” cold in the evening?

Yes. är states a static property; blir expresses change: Sovrummet blir kallt på kvällen = the bedroom gets cold in the evening.

Are both fönster and sovrum always neuter?

Yes: ett fönster and ett sovrum. Many compounds ending in -rum are neuter because rum is neuter and the compound inherits its gender. Fönster is also neuter and has identical indefinite singular/plural.

How would the sentence change in the plural?

Windows plural: Hennes fönster är stora, men …
Bedrooms plural: … men sovrummen är kalla på kvällarna. Note the plural adjectives (stora/kalla) and the plural time expression (på kvällarna).

Is kallt here an adjective or an adverb?

An adjective in predicative use, agreeing with neuter sovrummet. Some adjectives have -t forms that can also be adverbs, but here it clearly describes the subject (the room), so it’s adjectival.

Any quick pronunciation tips for tricky vowels?

ö in fönster ≈ rounded “e” sound.
ä in kvällen ≈ short open “e” sound.
Double l in kallt is short and crisp; pronounce the final t.