Learn Hardcore Norwegian: En liten regnbue speiler seg i elven rett ved torget. - A small rainbow reflects in the river right by the square.

Regnbue is a common-gender noun in Bokmål (sometimes called “utrum”). Most feminine and masculine nouns are grouped under the common gender and take en in the indefinite singular. The neuter article et would be used with neuter nouns (e.g. et hus), and ei is an alternative feminine article more typical in Nynorsk or dialect (e.g. ei bok), but standard Bokmål uses en regnbue.

Why is the adjective liten used here and not lite or lille?

In Norwegian, adjectives agree with the noun’s gender and definiteness. For an indefinite common-gender noun you use the weak form liten:

common gender (en-words) indefinite: liten regnbue
neuter (et-words) indefinite: lite hus
plural indefinite: små regnbuer
Lille is the weak form used before definite nouns (e.g. det lille huset) or in some set expressions (e.g. min lille venn).

Why is regnbue written as one word and not two separate words (regn bue)?

Norwegian generally forms compound nouns by merging two (or more) words into one. Here regn (“rain”) + bue (“arch”) combine to regnbue (“rainbow”). Writing them separately (regn bue) would be incorrect in Norwegian.

What does speiler seg mean, and why is the reflexive seg needed?

Å speile seg literally means “to mirror oneself” or “to be reflected.” The reflexive pronoun seg indicates that the subject (the rainbow) is both doing and receiving the action—i.e. it reflects itself in the water. In English we often say “is reflected,” but in Norwegian the reflexive construction is common with speile.

Could we use a passive verb like reflekteres instead of speiler seg?

Yes. A more formal or technical version is:
En liten regnbue reflekteres i elven rett ved torget.
Here reflekteres is the passive of reflektere (“to reflect”). But in everyday speech speiler seg is more idiomatic.

Why is it i elven and not på elven or i elva?

i elven (“in the river”) emphasizes that the reflection appears in the water itself.
på elven (“on the river”) would suggest something floating or moving on the surface (like a boat).
elva is simply the alternative definite form of elv; both elva and elven are correct in Bokmål. You choose one based on personal or regional preference.

What does rett ved mean, and can I use other expressions to say “right next to”?

Rett ved means “right next to” or “just by,” stressing immediate proximity. Other options:

akkurat ved (very similar)
lige ved (more common in Denmark, understood in Norway)
ved siden av (“beside” or “next to”), though you’d often add an adverb for emphasis: rett ved siden av.

Why is torget in the definite form instead of et torg?

Torget (“the square”) is definite because it refers to a specific, identifiable place (the town square). If you were introducing any square in general, you’d say et torg, but here both speaker and listener know which square is meant.

Could the word order change? For example, can I start the sentence with Rett ved torget?

Yes, you can front the location for emphasis:
Rett ved torget speiler en liten regnbue seg i elven.
Norwegian V2 word order requires the finite verb (speiler) to come second, but you can choose which element (subject, adverbial, etc.) comes first. The original order (subject–verb–adverbials) is simply the most neutral.

How do you pronounce regnbue, and where is the stress?

The primary stress is on the first syllable: REGN-bue. In IPA you’ll often see /ˈrɛŋˌbuːə/. The gn cluster is pronounced [ŋn] (a velar nasal followed by an alveolar nasal). There’s a lighter secondary stress on bue.