Learn Hardcore Norwegian: Hun har allergi mot støv, så hun åpner vinduet. - She has an allergy to dust, so she opens the window.

Yes—both are natural. "Hun er allergisk mot støv" (adjective) and "Hun har allergi mot støv" (noun) mean the same. You will also hear the compound "Hun har støvallergi." In everyday speech, "er allergisk mot" is probably the most common.

Why is the preposition "mot" used? Why not "til" or "for"?

With allergies, Norwegian uses mot: allergi/allergisk mot noe = allergic to something. "til" is not used here. "for" sometimes appears in colloquial speech but is not recommended in standard writing. You may also see allergisk overfor X; it’s acceptable but less common and often used figuratively (e.g., "allergisk overfor kritikk").

Why is it "vinduet" (the window) and not just "vindu" (a window)?

Norwegian uses the definite form when the thing is specific/known in context. Hun åpner vinduet implies the particular window in the room. If it’s any random window, say Hun åpner et vindu. With an adjective you get double definiteness: Hun åpner det store vinduet.

What gender is "vindu," and how do I inflect it?

"Vindu" is neuter:

Indefinite singular: et vindu
Definite singular: vinduet
Indefinite plural: vinduer
Definite plural: vinduene

Does "så" change the word order? Why "så hun åpner" and not "så åpner hun"?

In this sentence så is a coordinating conjunction meaning "so/therefore," linking two main clauses. After a coordinating conjunction, the next clause keeps normal subject–verb order: så hun åpner …. When så is an adverb meaning "then," it occupies the first position and triggers inversion (V2): Så åpner hun vinduet.

Is the comma before "så" required?

Yes. When så links two full main clauses, a comma before it is standard: Hun har …, så hun …. If så starts a new sentence as an adverb ("Then …"), you don’t use a comma before it.

Can I use "derfor" instead of "så"?

Yes: Hun har allergi mot støv; derfor åpner hun vinduet. Derfor is an adverb ("therefore"). Because it’s first in the clause, V2 word order puts the verb before the subject: derfor åpner hun …. Use a semicolon or a period before derfor, not just a comma.

What about "fordi" (because)?

Fordi introduces a subordinate reason clause:

Hun åpner vinduet fordi hun har allergi mot støv. (no comma needed before fordi)
Fordi hun har allergi mot støv, åpner hun vinduet. (comma after the fronted subordinate clause, and inversion in the main clause: åpner hun)

Why is there no article before "allergi"?

With illnesses/conditions, Norwegian often omits the article after ha: ha allergi, ha feber, ha influensa. Hun har en allergi mot … is grammatically possible but sounds unnatural; prefer har allergi or er allergisk.

Is "støv" countable? Can it be plural?

Støv (dust) is a mass noun. You normally use it without an article: støv. The definite form is støvet ("the dust"). A plural is not used in ordinary language.

What exactly does the verb form "åpner" express—"opens" or "is opening"?

Norwegian present tense covers both simple and progressive meanings. Hun åpner vinduet can mean either "She opens the window (habitually)" or "She is opening the window (now)." Morphology: present is usually formed by adding -r to the infinitive: å åpne → åpner. To emphasize an ongoing action, you can say Hun holder på å åpne vinduet (nå).

Is "åpner opp" okay, or is it redundant?

Åpne opp is common in speech and not wrong, but many style guides prefer plain åpne because opp adds little meaning here. Use åpne in neutral written style: Hun åpner vinduet.