Learn Hardcore Norwegian: Læreren sier at det er urettferdig å generalisere. - The teacher says that it is unfair to generalize.

At is a subordinating conjunction that introduces a content clause (a that‑clause) after verbs like si(er), mene, tro, etc.
So Læreren sier at det er urettferdig å generalisere literally is The teacher says that it is unfair to generalize.
Functionally, at here corresponds to English that.

Why do we need det in at det er urettferdig å generalisere? Can we say at er urettferdig å generalisere?

You cannot drop det here; at er urettferdig å generalisere is ungrammatical.
In det er urettferdig å generalisere, det is a dummy (formal) subject, like English it in It is unfair to generalize.
The real meaning chunk is the infinitive phrase å generalisere, but Norwegian (like English) normally inserts det / it as the grammatical subject in this pattern.

Could we also say Å generalisere er urettferdig? If yes, what is the difference?

Yes, Å generalisere er urettferdig is correct and means the same thing.
The difference is style and focus:

Det er urettferdig å generalisere is the most common, neutral pattern in Norwegian.
Å generalisere er urettferdig makes å generalisere (generalizing) more topical or emphasized, and sounds a bit more like a general rule or slogan.

In the original sentence, with sier at, both versions would work, but the det er … å … structure is more idiomatic.

What is the function of å before generalisere?

Å is the Norwegian infinitive marker, like to in English to generalize.
So å generalisere = to generalize.
Do not confuse å with og (and). Å is always followed by a verb in the infinitive; og links words or phrases.

Examples:

Jeg liker å lese. – I like to read.
Jeg liker å lese og skrive. – I like to read and write.

Why is it urettferdig and not something like urettferdigt or urettferdige?

Urettferdig is an adjective that ends in -ig, and most such adjectives have the same form in masculine and neuter singular when used like this.
In predicative position (after er, blir, etc.) with dummy det, you normally use the base form: det er urettferdig.
You would use urettferdig for all genders in the singular:

en urettferdig dom – an unfair verdict
ei urettferdig straff – an unfair punishment
et urettferdig system – an unfair system

The plural adds -e: urettferdige regler (unfair rules).

What is the difference between lærer and læreren?

Lærer = a teacher / teacher (indefinite).
Læreren = the teacher (definite; literally teacher‑the).
Norwegian often uses the en / ei / et ending on the noun itself to show definiteness, instead of a separate the word.

So:

En lærer sier at … – A teacher says that …
Læreren sier at … – The teacher says that …

Is the word order inside the at‑clause always like det er urettferdig (subject–verb), or can it be verb‑second like in main clauses?

In main clauses, Norwegian normally has V2 word order (the verb in second position):

Læreren sier dette. – The teacher says this.

In subordinate clauses introduced by at, the order is subject – verb (no V2):

Læreren sier at det er urettferdig … (subject det, then verb er).

So inside an at‑clause you generally keep subject–verb, not verb‑second.

Could we drop at and just say Læreren sier det er urettferdig å generalisere?

Yes, in spoken Norwegian and informal writing, at is often dropped after common verbs like si, synes, mene, tro.
So Læreren sier (at) det er urettferdig å generalisere is possible, and many natives will omit at in casual speech.
However, in careful or formal writing, it is more standard and clear to keep at.

Why is it sier and not snakker or forteller?

Norwegian has several verbs related to speaking, with different uses:

si (present sier) = to say. It introduces the actual words or content, often with at:
- Læreren sier at … – The teacher says that …
snakke = to speak / talk (a more general activity):
- Læreren snakker om rettferdighet. – The teacher talks about justice.
fortelle = to tell / narrate, often when giving information or telling a story, usually with an indirect object:
- Læreren forteller elevene at … – The teacher tells the students that …

In your sentence, we report what is being said, so sier is the natural choice.

How would this sentence change if læreren were plural, like the teachers say that …?

You would make lærer plural and definite: lærerne = the teachers.
The verb sier stays the same in the present tense (it does not change with number):

Lærerne sier at det er urettferdig å generalisere. – The teachers say that it is unfair to generalize.

Everything after at remains unchanged.

How do you pronounce æ in læreren and the g in generalisere?

Æ in læreren is like the vowel in English cat, but often a bit longer and more fronted. Læreren is roughly [ˈlæːrərən].
In generalisere, the g is a soft g, like in English get but often weaker; the word is roughly [ɡənɛrɑliˈseːrə] in many accents.

Exact pronunciation varies by dialect, but these approximations will be understood.