Learn Swedish: Elden spred sig till nästa hus. - The fire spread to the next house.

In Swedish, the verb sprida (to spread) requires an object. When something spreads on its own, like fire, a rumor, or a disease, you must use the reflexive pronoun sig. Without sig, the sentence would sound incomplete, as if the fire were spreading a substance like butter.

Why is it nästa hus (indefinite) and not nästa huset (definite)?

Even though nästa refers to a specific house, it is an exception to the usual Swedish rule for adjectives. The word nästa is almost always followed by a noun in the indefinite form (hus). If you said nästa huset, it would sound grammatically incorrect to a native speaker.

Is spred an irregular verb? What are its other forms?

Yes, sprida is an irregular strong verb. The forms you need to know are sprida (infinitive), sprider (present), spred (past), and spridit (supine). You might occasionally see the alternative past tense form spridde, but spred is the standard choice here.

Could I say branden instead of elden in this context?