Learn Hardcore Finnish: Sauna on kuuma, mutta rentouttava.

Finnish doesn’t have words equivalent to a, an, or the. You simply say Sauna on kuuma to mean The sauna is hot (or A sauna is hot, depending on context). Definiteness and indefiniteness are understood from context, not from articles.

What does on mean in this sentence?

On is the 3rd person singular present tense of the verb olla, meaning to be. So Sauna on translates as The sauna is.

Why are kuuma and rentouttava in their base (nominative singular) forms?

They are predicate adjectives describing sauna. In Finnish, predicate adjectives agree in case and number with the subject. Since sauna is nominative singular, both adjectives are nominative singular as well.

What kind of adjective is rentouttava, and how is it formed?

Rentouttava is formed with the suffix -tava (or -ttava) from the verb rentouttaa to make someone relax. The -tava suffix turns verbs into adjectives meaning causing or capable of doing that action. So rentouttava literally means causing relaxation, i.e. relaxing.

Why not use rentoutunut instead of rentouttava?

Rentoutunut is the past participle of rentoutua to relax oneself, meaning relaxed. It describes someone who has become relaxed. Rentouttava, on the other hand, describes something that makes you relax. A sauna can be relaxing, but it can’t itself be relaxed.

Why is mutta used rather than ja?

Mutta means but and introduces a contrast. Here you contrast the sauna’s high temperature (hot) with its positive effect (relaxing). Using ja (and) would simply list two qualities without highlighting that contrast.

Why is there a comma before mutta?

Finnish punctuation separates two independent clauses with a comma before conjunctions like mutta, much like in English. So you write Sauna on kuuma, mutta rentouttava.