Fjällstugan var liten men varm.

Because Swedish often makes compound nouns by joining words together.

Here, fjäll means mountain / fell and stuga means cabin / cottage. Together, fjällstuga means mountain cabin.

So instead of writing something like mountain cabin as two words, Swedish usually combines them into one: fjällstuga.

The -n is the definite ending, meaning the.

• en stuga = a cabin
• stugan = the cabin
• en fjällstuga = a mountain cabin
• fjällstugan = the mountain cabin

So Swedish often adds definiteness to the end of the noun instead of using a separate word.

Because in Swedish, the definite article is often attached to the noun as a suffix.

In English:

• the mountain cabin

In Swedish:

• fjällstugan

That final -n already means the, so you do not need a separate article here.

Var is the past tense of är (is / are).

So:

• är = is / are
• var = was / were

In this sentence, var means was because the sentence is talking about the cabin in the past:

• Fjällstugan var liten men varm = The mountain cabin was small but warm

A useful point: Swedish verbs do not change depending on the subject.

• jag var = I was
• du var = you were
• vi var = we were

The form var stays the same.

Because stuga is an en-word (common gender), and in this kind of sentence the adjective agrees with that gender.

Swedish adjectives often change form:

• common gender singular: liten
• neuter singular: litet
• plural: små

Since fjällstugan is singular and based on en stuga, the correct form here is liten.

Compare:

• stugan var liten = the cabin was small
• huset var litet = the house was small

This is a very common question.

Lilla is used when the adjective comes before a definite noun:

• den lilla fjällstugan = the small mountain cabin

But in your sentence, the adjective comes after the verb var. This is called predicative use, and then Swedish uses a different form:

• Fjällstugan var liten = The mountain cabin was small

So:

• den lilla fjällstugan = adjective before the noun
• fjällstugan var liten = adjective after var

For the same reason as liten: the noun is singular and common gender (en stuga), so the adjective takes the common-gender singular form.

• varm = common gender singular
• varmt = neuter singular
• varma = plural

Compare:

• stugan var varm = the cabin was warm
• huset var varmt = the house was warm

Men means but.

It connects two contrasting descriptions:

• liten = small
• varm = warm

So the sentence means that the cabin was small, but despite that, it was warm.

Yes, very similar.

The basic structure is:

• Fjällstugan = subject
• var = verb
• liten men varm = adjective phrase

So it matches the English pattern:

• The mountain cabin was small but warm

This is a straightforward Swedish main clause, so it feels very familiar to an English speaker.

You would change var to är:

• Fjällstugan är liten men varm.

That means The mountain cabin is small but warm.

Everything else stays the same.

You would use the indefinite form of the noun:

• En fjällstuga var liten men varm.

Compare:

• en fjällstuga = a mountain cabin
• fjällstugan = the mountain cabin

So the main difference is whether the noun is indefinite (en fjällstuga) or definite (fjällstugan).