Learn Swedish: Under strömavbrottet smälte smöret. - During the power outage, the butter melted.

In Swedish, under is the standard preposition for "during" when referring to a period of time or an event. So under strömavbrottet translates naturally to "during the power outage."

Why is the verb smälte placed before the subject smöret?

This is the classic V2 (verb-second) rule. Since the sentence starts with the prepositional phrase under strömavbrottet (which takes up the first position), the verb smälte must immediately follow in the second position, pushing the subject smöret to the third position.

What are the root words that make up strömavbrottet?

It is a compound of ström (power or electrical current) and avbrott (break or interruption). The -et at the very end is the definite article suffix for an ett word, making it "the power outage."

Is the past tense of smälta always smälte? I thought I once saw smalt.

In older Swedish, smalt was used when something melted on its own (intransitive), while smälte was used when someone actively melted something (transitive). Today, that distinction is mostly gone in everyday speech, and smälte is the accepted standard past tense for both situations.