Learn Hardcore Norwegian: Under strømbruddet var det stille i huset. - During the power outage it was quiet in the house.

Here Under is a temporal preposition meaning “during.” It does not mean “beneath” in this context. Norwegian often uses under for events that span a period of time. You could also say i løpet av strømbruddet, but under is more concise.

Why is strømbruddet in the definite form?

Strømbrudd (“power outage”) is a neuter noun. To refer to a specific outage, you make it definite by adding -et, yielding strømbruddet (“the power outage”). The indefinite form would be et strømbrudd (“a power outage”).

Why does the verb var come before det? Shouldn’t the subject come first?

Norwegian main clauses follow the “V2” (verb-second) rule. If you put an adverbial phrase (Under strømbruddet) in front, the finite verb (var) must come next, pushing the subject (det) to the third position. A neutral word order version is:
Det var stille i huset under strømbruddet.

Why is det used here as the subject? I thought “the house” was the subject.

This det is a dummy (expletive) subject, similar to English “it” in “It was quiet.” Grammatically, det doesn’t refer to the house; it just fills the subject slot so you can predicate “stille”.

Why isn’t stille inflected? Should it have an –t or –e somewhere?

In predicate position after være (“to be”), adjectives in Norwegian typically remain in their basic (indefinite) form. Moreover, stille is indeclinable—it looks the same regardless of gender and number. So you simply say det var stille.

Could I rearrange the sentence differently?

Yes. If you don’t front the adverbial, you follow a normal subject–verb–object order:
Det var stille i huset under strømbruddet.
You could also say:
Under strømbruddet var huset stille.
but then you’d be focusing more on huset (“the house”) as the topic rather than the quiet.