Learn Hardcore Finnish: Koulussa on sääntö, että puhelin pidetään äänettömällä tunnin aikana. - At school there is a rule that the phone is kept on silent during the lesson.

There is also koululla (adessive, at/by the school), but that usually refers to the building/yard as a physical location (e.g. Let’s meet at the school building), not to the institution in general. For a general school rule, koulussa is the normal choice.

What does että do here? Could you leave it out like English sometimes leaves out that?

Että is a conjunction meaning that, introducing a subordinate clause:

on sääntö, että … = there is a rule that …

In English you can often drop that:
There is a rule (that) the phone must be kept on silent…

In Finnish you cannot normally drop että in this kind of sentence. After nouns like sääntö (rule), päätös (decision), tieto (information), and many verbs, että is required:

On sääntö, että puhelin pidetään äänettömällä. ✅
On sääntö, puhelin pidetään äänettömällä. ❌ (ungrammatical)

So here että is obligatory.

Why is puhelin singular with no article, even though the rule applies to all phones?

Finnish has no articles (a, an, the), and it often uses the singular to talk about things in general.

Puhelin can mean:

a phone
the phone
phone(s) (in general)

The exact meaning comes from context. Here we’re talking about a general rule at school, so puhelin is understood as any student’s phone / phones in general.

You could also say:

Koulussa on sääntö, että puhelimet pidetään äänettömällä…
= there is a rule that phones are kept on silent…

That sounds a bit more explicitly plural but the original puhelin is completely natural for a general rule.

Why is the passive form pidetään used? Who is doing the action?

Pidetään is the so‑called Finnish passive (often called impersonal). It has no explicit subject and corresponds roughly to English is kept or people keep.

puhelin pidetään äänettömällä
= the phone is kept on silent
= people keep the phone on silent

This is very typical in rules, instructions, and general statements:

Koulussa puhutaan suomea.
Finnish is spoken at school / people speak Finnish at school.
Ovet suljetaan kello kahdeksan.
The doors are closed at eight.

If you used an active form, you would need a subject:

Oppilaat pitävät puhelimen äänettömällä.
The students keep the phone on silent.

But in a rule text, the passive pidetään is more neutral and formal.

What does the phrase pidetään äänettömällä literally mean?

The verb here is pitää in the sense to keep, to hold (in some state).

Structure:

pitää + object + (state/setting in a case form)
puhelin pidetään äänettömällä
literally: the phone is kept on silent (setting)

Äänettömällä is adessive (more on that below) and refers to the phone’s silent mode / silent setting. Finnish often uses olla/pitää + adessive to talk about device settings, channels, modes:

TV on kakkosella.
The TV is on channel two.
Puhelin on äänettömällä.
The phone is on silent.

So pidetään äänettömällä = is kept on the silent setting.

Why is äänettömällä in the adessive case (-lla) instead of some form like äänetön or äänettömänä?

Äänetön is an adjective meaning silent, without sound. When we talk about device modes or settings, Finnish often uses the adessive case (-lla/-llä) to express on X mode/channel:

äänettömällä = on silent (mode)
värinässä / värinällä = on vibrate (both exist colloquially)
ykkösellä / kakkosella = on channel one / two

You can think of äänettömällä as a shortened form of:

äänettömällä tilalla / asetuksella
on the silent mode/setting

Other forms would mean something else:

äänetön (basic form) – just the adjective, not used alone here.
äänettömänä (essive) – being silent, usually about a person or thing’s state, not a phone’s mode.

So puhelin pidetään äänettömällä specifically matches the idea the phone is kept on silent mode.

What exactly does tunnin aikana mean, and why is it tunnin (genitive)?

tunti = hour; lesson (class period)
aikana = during

When aikana is used, the word before it goes into the genitive:

tunnin aikana = during the lesson / during the class
kesän aikana = during the summer
vuoden aikana = during the year

So tunnin is simply the genitive of tunti, required by aikana.

The phrase tunnin aikana suggests throughout the duration of the lesson, not just at some point in it.

Could we say tunnilla instead of tunnin aikana? What’s the difference?

Yes, you can say both, but the nuance changes slightly:

tunnilla (adessive)
= in class, during (a) lesson in a more general, locational sense
Kännykät ovat äänettömällä tunnilla.
Phones are on silent in class.
tunnin aikana
= during the lesson, emphasising the time span / whole duration
Kännykät pidetään äänettömällä tunnin aikana.
Phones are kept on silent during the lesson (for the whole class period).

In practice, both could appear in a school rule. Tunnin aikana sounds a bit more formal and explicitly temporal.

Why is it tunnin in the singular when the rule applies to every lesson?

Finnish often uses the singular to express a generic repeated situation:

Tunnin aikana = during a (any) lesson
Koulupäivän aikana = during the school day (each school day)

The idea is: whenever there is a lesson, during that lesson the rule applies. English often does the same with the or a in a generic sense (during the lesson, during class).

You could make it explicitly plural:

Kaikkien tuntien aikana (during all lessons)

but that sounds heavier and less natural as a general rule. The simple singular tunnin aikana already suggests for any lesson.

The English translation has a sense of must (the phone must be kept on silent). Where is that must in the Finnish sentence?

There is no separate word for must here. The obligation comes from the context and structure:

Koulussa on sääntö, että…
There is a rule that…

When you state something as the content of a rule, it is automatically understood as something you must do. Finnish often uses just the passive present for rules:

Tunnilla ei syödä.
You don’t eat in class / Eating in class is not allowed.
Kengät jätetään eteiseen.
Shoes are left in the hallway.

If you wanted to make the obligation explicit and more formal, you could say:

Puhelin on pidettävä äänettömällä tunnin aikana.
literally: The phone is to be kept on silent during the lesson.

But the original sentence already clearly expresses a rule.

Why is the word order puhelin pidetään äänettömällä and not pidetään puhelin äänettömällä?

Neutral Finnish word order in such a clause is Subject – Verb – the rest:

Puhelin pidetään äänettömällä.
The phone is kept on silent.

Here, puhelin is the logical subject (the thing affected), even though the verb is passive.

You can say Pidetään puhelin äänettömällä, but:

It sounds more like an instruction you might say out loud: Let’s keep the phone on silent.
It puts more emphasis on the action (pidetään) rather than on the phone.

In a written rule describing what happens to the phone, Puhelin pidetään äänettömällä is the more typical, neutral word order.

Can you show a few alternative ways to phrase the same rule in Finnish, including something more informal?

Sure. All of these convey roughly the same idea:

Koulussa on sääntö, että puhelin pidetään äänettömällä oppitunnin aikana.
– Adds oppitunnin (class lesson), a bit more formal/precise.
Koulussa on sääntö: puhelin on äänettömällä tunnin aikana.
– Uses on äänettömällä (is on silent) instead of *pidetään äänettömällä; slightly more descriptive.
More direct / informal, as a teacher might say to students:
Tunnilla puhelimet pitää olla äänettömällä.
– puhelimet (plural), pitää olla = have to be.
Very formal rulebook style:
Puhelin on pidettävä äänettömällä oppitunnin ajan.
– on pidettävä clearly marks obligation (must be kept).

What cases appear in koulussa on sääntö, että puhelin pidetään äänettömällä tunnin aikana, and what does each contribute?

Key case forms:

koulussa – inessive (-ssa/-ssä)
in/at school (location within the institution)
äänettömällä – adessive (-lla/-llä)
on silent (setting); adessive here expresses being on a certain mode/channel
tunnin – genitive (-n)
of the lesson; required by aikana
aikana is an adverbial form (from aika, time) that means during; the word before it takes the genitive: tunnin aikana (during the lesson).

Together, these case choices let Finnish express relationships (location, state, time span) without prepositions, where English uses at, on, during.