Learn Hardcore Norwegian: Dagen da skolen forbyr telefoner blir roligere. - The day when the school forbids phones becomes calmer.

Here da is a relative adverb meaning when/on which, linking the clause to dagen. You can’t use når after a noun to make this kind of relative clause.

Correct: Dagen da skolen forbyr telefoner …
Separate adverbial clause: Når skolen forbyr telefoner, … (means whenever/when)

Could I write Dagen som skolen forbyr telefoner?

No. som stands in for a subject or object inside the relative clause. Here, dagen corresponds to a time adjunct (when), not a subject/object, so you need the relative adverb da, not som.

Is Den dagen … better than Dagen …?

Both are possible. Den dagen … adds a bit of pointing/emphasis to that particular day. Dagen … is more neutral. Many speakers prefer Den dagen … if they then use an impersonal main clause like … blir det roligere.

Do I need a comma before da or before blir?

No comma before da in a restrictive relative clause.
In the given sentence, no comma is needed.
If you start with a subordinate clause like Når skolen forbyr telefoner, put a comma before the main clause: Når skolen forbyr telefoner, blir det roligere.
After a long fronted time phrase like Den dagen da skolen forbyr telefoner, a comma is optional (stylistic).

Why is it blir roligere and not er roligere?

bli marks a change of state (becomes). være/er simply describes a state (is). Use blir if the calmness is a result of the ban taking effect; use er if you just state that the day is calmer.

Is the word order right? I thought verbs go in second position (V2).

Yes. The whole subject Dagen da skolen forbyr telefoner is position 1. The finite verb blir is in position 2. That’s exactly V2.

How does forby conjugate?

Infinitive: å forby
Present: forbyr
Preterite: forbød (also forbydde, less common)
Perfect participle: forbudt (har forbudt) Related noun: et forbud (a ban).

Should it be forbyr telefoner or forbyr bruk av telefoner / forbyr å bruke telefoner?

All are fine, with nuances:

forbyr telefoner = bans the devices themselves.
forbyr bruk av telefoner / forbyr å bruke telefoner = bans the use of phones. In school talk, forbyr mobiltelefoner or forbyr mobilbruk is very common.

Why is telefoner in the indefinite plural and not telefonene?

Indefinite plural (telefoner) is used for generic reference (phones as a category). Definite plural (telefonene) would refer to specific, identifiable phones.

Would mobiltelefoner or mobiler be more natural than telefoner?

Often yes, if you mean cell phones specifically:

mobiltelefoner (neutral/formal)
mobiler (colloquial plural of en mobil) telefoner is broader and can include landlines.

What’s the difference between roligere and stillere?

roligere = calmer, more tranquil (mood/behavior/atmosphere).
stillere = quieter, less sound/noise. Choose based on what you want to highlight.

Can I say mer rolig instead of roligere?

You can, but with adjectives like rolig, the comparative ending -ere is preferred: roligere. Superlative: roligst/roligste (use -e when definite or before a noun: den roligste dagen).

Is the sentence idiomatic, or would Norwegians phrase it differently?

It’s grammatical. For a habitual/general statement many would say:

På dager da skolen forbyr telefoner, er det roligere.
Når skolen forbyr telefoner, blir det roligere. For one specific day, your original works.

Is da only for the past? The verb here is present.

This da is a relative adverb meaning when/on which, not the past-time conjunction da used to introduce past events. It’s fine with present tense here.

Can I move the parts around?

Yes. Examples:

Det blir roligere den dagen skolen forbyr telefoner.
Den dagen skolen forbyr telefoner, blir det roligere.
Skolen blir roligere den dagen den forbyr telefoner. Just keep the main-clause verb in second position.

Why not på skolen to mean at school?

Here skolen is the subject (the school bans phones). If you want a location adverbial, you can say:

På skolen er det roligere den dagen de forbyr telefoner.
Når telefoner er forbudt på skolen, blir det roligere.

What are the basic forms of skole and telefon?

en skole – skolen – skoler – skolene
en telefon – telefonen – telefoner – telefonene