Learn Swedish: Smöret mjuknar om du värmer det. - The butter softens if you heat it.

Mjuknar comes from the adjective mjuk (soft). By adding the suffix -na, you create an inchoative verb, which is a grammar term for a verb that describes a change of state. So mjukna simply means to become soft or to soften.

Why don't we use a -na verb for heating the butter?

The -na verbs like mjukna are intransitive, meaning they describe a change happening to the subject itself without a direct object (the butter softens). Värmer (from värma) is used here because it is a transitive verb, meaning you are actively performing the action on an object (you heat the butter).

The word for warm is varm, so why is the verb spelled värmer with an ä?

When deriving verbs from adjectives, Swedish often changes the root vowel. An a in the adjective frequently shifts to an ä in the verb. You will see this vowel shift pattern in other word pairs too, such as varm (warm) becoming värma (to heat).

Why is the word det used at the end of the sentence?

Det means it and refers back to smöret (the butter). Because smör is an ett-word, the object pronoun must match its gender, so we use det instead of den.