Learn Hardcore Norwegian: Læreren mener at mobilbruken i timen forstyrrer alle. - The teacher thinks that the mobile phone use in class disturbs everyone.

mene (present: mener)
- Means to be of the opinion that, often a bit more considered, sometimes with a sense of judgment or evaluation.
- Læreren mener at … = The teacher is of the (considered) opinion that …
synes
- More like to feel / to think based on personal experience.
- Jeg synes filmen er bra. = I think/feel the movie is good (my personal impression).
tro (present: tror)
- To believe, often without direct evidence or experience.
- Jeg tror han kommer. = I believe he’s coming / I think he will come.

Here, mener fits well because the teacher is giving a reasoned opinion or judgment about phone use in class.

What is the role of at in this sentence? Can we leave it out?

at is a subordinating conjunction meaning that. It introduces the clause that functions as the object of mener:

Læreren mener [at mobilbruken i timen forstyrrer alle].
= The teacher thinks [that mobile phone use in class disturbs everyone].

About leaving it out:

In everyday spoken Norwegian, at is often dropped when the meaning is clear:
Læreren mener mobilbruken i timen forstyrrer alle.
This is common and generally understood as correct.
In careful or formal writing, it is safer and more standard to keep at.

So: yes, you can omit it in many contexts, but the version with at is the “safe” standard.

Why is it mobilbruken and not mobilbruk?

Again, this is definiteness:

mobilbruk = mobile(phone) use, phone usage (indefinite)
mobilbruken = the mobile(phone) use / the phone usage (definite)

Norwegian often uses the definite form when referring to a specific, known phenomenon in a specific context:

bilkjøring i byen = driving in a city (in general)
bilkjøringen i byen = the driving in the city (the specific, concrete situation)

Here, mobilbruken i timen points to the actual use of phones in that lesson setting — more concrete and specific than abstract “phone usage” in general.

Does mobilbruken mean use of one mobile phone, or mobile phones in general?

Even though mobilbruken is grammatically singular (literally something like the mobile-use), in practice it refers to mobile use in general in that setting, not just one phone.

Norwegian often uses a singular, definite abstract noun to talk about a general practice:

røyking på skolen / røykingen på skolen = smoking at school (as an activity)
mobilbruken i timen = the use of mobile phones in class (as a general behaviour)

So you should understand it as mobile phone use (as a practice) during the lesson, not the use of one specific phone.

What exactly does timen mean in i timen, and why not just i klassen or i løpet av timen?

en time in Norwegian often means a lesson / a class period, not just “time” in the abstract.

en time = a (school) lesson, a class; also an hour
timen = the lesson / the class (period)

i timen therefore means in class or during the lesson.

Alternatives:

i klassen = in the class (more about the group of students)
i løpet av timen = during the lesson (more explicitly “over the course of the lesson”)

i timen is the standard, compact way of saying in class / during the lesson.

Why is the word order mobilbruken i timen forstyrrer alle in the at-clause? Could it be something like forstyrrer mobilbruken i timen alle?

In a clause introduced by at, Norwegian does not use V2 word order. The usual neutral pattern is:

Subject – (adverbials) – Verb – (objects etc.)

In the sentence:

Subject: mobilbruken
Adverbial: i timen
Verb: forstyrrer
Object: alle

So the clause is:

mobilbruken i timen forstyrrer alle

The word order *forstyrrer mobilbruken i timen alle is incorrect in standard Norwegian.
The verb forstyrrer must come after the subject in this kind of subordinate clause.

Can i timen be moved? For example: … at mobilbruken forstyrrer alle i timen?

Yes. Both of these are grammatically fine:

… at mobilbruken i timen forstyrrer alle.
… at mobilbruken forstyrrer alle i timen.

The difference is subtle:

Version 1 slightly highlights the setting: phone use in class is the subject that disturbs everyone.
Version 2 sounds a bit more like everyone is disturbed in class (as opposed to somewhere else), but in context they will usually be understood the same.

Both word orders are natural in everyday Norwegian.

What does forstyrrer mean here, exactly? How is it different from plager or avbryter?

forstyrre (present: forstyrrer)
- Main meanings: to disturb, to interfere with, to disrupt concentration.
- Focus on making it hard for someone to focus or for something to function smoothly.
plage (present: plager)
- To annoy, to bother, to torment.
- Stronger sense of causing discomfort or suffering, not just distraction.
avbryte (present: avbryter)
- To interrupt, to cut something off.
- Used more for stopping someone who is speaking, or cutting an activity short.

In this context, forstyrrer is the natural choice because the idea is that phone use disturbs the lesson / everyone’s concentration, not necessarily that it annoys them (plager) or interrupts someone mid-sentence (avbryter).

How is forstyrrer conjugated? What is the infinitive and the past tense?

The verb is å forstyrre (to disturb). Basic forms in Bokmål:

Infinitive: å forstyrre
Present: forstyrrer
Preterite (past): forstyrret
Present perfect: har forstyrret
Imperative: forstyrr!

Examples:

Mobilbruken i timen forstyrrer alle.
Phone use in class disturbs everyone.
Mobilbruken i går forstyrret timen.
Phone use yesterday disturbed the lesson.
Det har forstyrret undervisningen lenge.
It has disturbed the teaching for a long time.

Could we say Læreren mener at mobilbruk i timene forstyrrer alle? How would that change the meaning?

Yes, that is a perfectly good sentence, but with a slightly different nuance:

mobilbruken i timen
- definite singular: “the phone use in class (in this type of setting / in that class)”
- Feels a bit more concrete and specific.
mobilbruk i timene
- mobilbruk: indefinite (phone use, as a general phenomenon)
- i timene: plural definite “in the lessons / in classes (generally)”
- Feels more like a general statement about phone use across lessons.

So your version stresses phone use in lessons in general, while the original easily reads as the current phone use in class (as we know it), though Norwegian definite singular can also express a general phenomenon. Context decides the exact feel.

Why is it alle and not alle sammen? Would alle sammen be wrong?

Both can work, but there is a nuance:

alle = everyone / all of them
- Neutral, works in all registers.
alle sammen = literally “all together / all of them together”
- More colloquial and a bit more emphatic: every single one of them.

In this sentence:

forstyrrer alle = disturbs everyone
forstyrrer alle sammen = disturbs absolutely everyone (slightly stronger, more informal)

alle is the more neutral, school‑book choice. alle sammen is not wrong; it just sounds a bit more conversational or emphatic.