Learn Hardcore Norwegian: Mekanikeren sier at vinterdekkene må skiftes før den første snøstormen. - The mechanic says that the winter tyres must be changed before the first snowstorm.

In the sentence, we are talking about a specific, known mechanic (for example, my mechanic or the one at the workshop), so Norwegian uses the definite form Mekanikeren with -en attached.

What is the function of at in sier at vinterdekkene må skiftes? Do we need it?

Here at is a subordinating conjunction meaning that. It introduces a subordinate clause describing what the mechanic says.

Mekanikeren sier at … = The mechanic says that …

In English, that is often dropped:

The mechanic says (that) the winter tires must be changed.

In Norwegian, you usually cannot drop at in this kind of sentence.
So:

Correct: Mekanikeren sier at vinterdekkene må skiftes.
Incorrect: ✗ Mekanikeren sier vinterdekkene må skiftes.

Why is the word order in the at-clause vinterdekkene må skiftes and not verb-second like in main clauses?

Norwegian has verb-second (V2) word order in main clauses, but not in subordinate clauses.

Main clause: Vinterdekkene må skiftes.
(Subject – verb – other elements; verb is in second position.)
Subordinate clause after at: at vinterdekkene må skiftes
Here the pattern is typically: [subordinator] + subject + verb + …

So:

Mekanikeren sier [at vinterdekkene må skiftes].

You do not move the verb to second position after at; it stays after the subject.

What exactly does må mean here? Is it more like must, have to, or should?

Må expresses necessity/obligation, and is strongest translated as must or have to.

vinterdekkene må skiftes
≈ the winter tires must be changed / have to be changed

Depending on context and tone, it can feel slightly softer, like strong advice, but grammatically it is an obligation, stronger than bør (should).

Quick comparison:

må = must / have to
skal = shall / is supposed to (plan, duty, or future)
bør = should (recommendation)

Why is it skiftes and not skifte or bli skiftet?

Skiftes is the passive infinitive form of å skifte (to change).

Norwegian has two main ways to form the passive:

s-passive (with -s):
- vinterdekkene må skiftes
  = the winter tires must be changed.
bli + past participle:
- vinterdekkene må bli skiftet
  = literally the same meaning: must be changed.

Both are grammatically correct here. Skiftes is shorter and very common in written and formal language, especially with modal verbs like må.

You would not say må skifte in this context, because that would be active:

Mekanikeren må skifte vinterdekkene.
= The mechanic must change the winter tires. (Mechanic is the one doing the action.)

Why is there no å before skiftes? Shouldn’t it be å skiftes?

After modal verbs like må, kan, vil, skal, Norwegian does not use å before the infinitive.

Jeg må gå. (not må å gå)
Hun kan lese. (not kan å lese)
Dekkene må skiftes. (not må å skiftes)

So må skiftes is the correct pattern: [modal verb] + [infinitive without å].

What is the difference between vinterdekk and vinterdekkene?

Vinterdekk is a neuter noun:

et vinterdekk = a winter tire
vinterdekket = the winter tire
vinterdekk = winter tires (indefinite plural)
vinterdekkene = the winter tires (definite plural)

In the sentence:

vinterdekkene = the winter tires (a specific set, probably the ones on the car).

If you said just vinterdekk here, it would sound more general, like winter tires as a concept, not the particular ones that need changing.

Why is it den første snøstormen? Why both den and -en?

Norwegian often uses double definiteness with adjectives:

Pattern:

den
- adjective + definite noun

With snøstorm (snowstorm):

en snøstorm = a snowstorm
snøstormen = the snowstorm
den første snøstormen = the first snowstorm

So:

den = definite article before the adjective
snøstormen = definite form of the noun (with -en)

Both are needed; you cannot drop one of them here.

Correct: før den første snøstormen
Incorrect: ✗ før første snøstormen or ✗ før den første snøstorm

What is the difference between før and første in this sentence?

They are completely different words:

før = before (preposition/conjunction of time)
første = first (ordinal number, like English first)

In the sentence:

før tells you when: before something.
den første snøstormen is the event: the first snowstorm.

So the phrase is:

før den første snøstormen = before the first snowstorm.

Could I say før første snøstorm instead of før den første snøstormen?

Not in normal, standard Norwegian.

Because there is an adjective (første) plus a specific, countable noun (snøstorm), Norwegian wants the double definite form:

før den første snøstormen (correct)

Før første snøstorm sounds incomplete or non-native in standard Bokmål. You might see bare forms in headlines or very telegraphic styles, but in normal sentences you should use den … -en.

Why is snøstormen in the definite form if we already have den?

This is exactly the double definiteness rule with adjectives:

Without adjective:
- snøstormen = the snowstorm
With adjective:
- den store snøstormen = the big snowstorm
- den første snøstormen = the first snowstorm

So when there is an adjective (første, store, kraftige, etc.), you keep both:

den (article before the adjective)
snøstormen (definite noun form)

You cannot change only one of them to indefinite; they work together.

Could I change the word order and say something like Før den første snøstormen må vinterdekkene skiftes, sier mekanikeren?

Yes. That sentence is grammatical and natural:

Før den første snøstormen må vinterdekkene skiftes, sier mekanikeren.
= Before the first snowstorm, the winter tires must be changed, the mechanic says.

Here:

Før den første snøstormen is moved to the front as a time expression.
In the main clause after that, the verb må still appears in second position (V2):
- [Time] – må – vinterdekkene – skiftes

So several word orders are possible as long as you respect:

V2 in main clauses
No V2 after at in subordinate clauses

Does vinterdekk behave like an uncountable noun (like furniture) or a countable one?

Grammatically, vinterdekk is countable and neuter, but its singular and plural indefinite forms look the same:

Singular: et vinterdekk (a winter tire)
Definite singular: vinterdekket (the winter tire)
Indefinite plural: vinterdekk (winter tires)
Definite plural: vinterdekkene (the winter tires)

So vinterdekk without article can mean winter tires in general (plural), but context usually makes it clear whether it’s singular or plural. In this sentence, the definite plural vinterdekkene clearly refers to the tires on the car.