Learn Hardcore Norwegian: Lyden blir høyere når vi skrur på musikken. - The sound becomes louder when we turn on the music.

In this sentence, Lyden blir høyere, we are talking about the sound in the room / on the device, something both speaker and listener can identify. That is why the definite form lyden is used, not the bare noun lyd.

You could say:

Lyd blir høyere – grammatically odd and not natural here
Lyden blir høyere – natural: The sound gets louder

Why do we use blir and not er in Lyden blir høyere?

Blir (from å bli) means becomes / gets, and it emphasizes a change from one state to another.

Lyden er høy. – The sound is loud. (describes a state)
Lyden blir høyere. – The sound is getting louder / becomes louder. (describes a change)

In your sentence, the sound is changing when we turn on the music, so Norwegian uses blir to show this process of becoming louder.

Why is it høyere and not just høy or mer høy?

Høyere is the comparative form of the adjective høy:

høy = high / loud
høyere = higher / louder
høyest = highest / loudest

So:

Lyden er høy. – The sound is loud.
Lyden blir høyere. – The sound gets louder.

Saying mer høy is technically understandable but not idiomatic here. Norwegian normally uses the -ere ending for most one‑syllable adjectives (and many two‑syllable ones):

stor → større (big → bigger)
lang → lengre (long → longer)
høy → høyere (high/loud → higher/louder)

So høyere is the natural comparative form.

I learned that høy means tall or high. How can it also mean loud in høyere?

Norwegian høy covers several meanings that English splits into different words:

Tall – for people or objects
- Han er høy. – He is tall.
High – for positions/levels
- Temperaturen er høy. – The temperature is high.
Loud – for volume/sound level
- Musikken er høy. – The music is loud.
- Lyden blir høyere. – The sound gets louder.

So when høy refers to sound, music, volume, it is best translated as loud, and høyere as louder.

Why is the present tense skrur, not something like skruer?

The verb is å skru (to turn, to screw, to turn on).
Its present tense is skrur, not skruer.

This is a regular pattern for a group of short verbs ending in a vowel:

å tro → tror (to believe)
å bo → bor (to live, reside)
å snu → snur (to turn around)
å skru → skrur (to turn / to turn on)

So the correct conjugation is:

å skru – infinitive
jeg skrur – I turn / I turn on
vi skrur – we turn / we turn on

Skruer exists as a noun (plural of skrue = screw), but not as the present tense of å skru.

What does skru på literally mean, and how is it different from slå på or sette på?

Literally, å skru på is to screw/turn on (think of turning a knob). In practice, it just means to turn on (a device, the sound, etc.).

Common phrasal verbs:

å skru på – to turn on (often with the idea of turning a knob or button)
å slå på – to switch on (pressing a switch, a button, power)
å sette på – to put on / start playing (music, a film, a program)

In many everyday contexts, they overlap:

Skru på lyden. – Turn on the sound.
Slå på TV-en. – Turn on the TV.
Sett på musikk. – Put some music on / start the music.

In your sentence når vi skrur på musikken, it can mean both turn on the music and turn up the music depending on context, but usually it is understood as turn on.

Why is it musikken (definite) instead of just musikk?

As with lyden, the -en ending makes musikk definite:

musikk = music (in general)
musikken = the music (specific, known)

Når vi skrur på musikken usually suggests a particular source or set of music:
the music on the stereo, on the phone, on the system the speaker has in mind.

Both are possible, but they sound a bit different:

når vi skrur på musikk – when we turn on music (any music, more general)
når vi skrur på musikken – when we turn on the music (the usual/known music)

The definite form fits naturally if both speaker and listener know what music they mean.

Can I put the når‑clause first: Når vi skrur på musikken, blir lyden høyere?

Yes, absolutely. Both word orders are correct:

Lyden blir høyere når vi skrur på musikken.
Når vi skrur på musikken, blir lyden høyere.

The meaning is the same.
The main thing to remember is word order after a fronted når‑clause:

Subordinate clause (Når vi skrur på musikken)
Finite verb of the main clause (blir)
Subject of the main clause (lyden)

So you must say:

Når vi skrur på musikken, blir lyden høyere. ✅
Not: Når vi skrur på musikken, lyden blir høyere. ❌

What is the difference between når and da? Could I use da here?

Both når and da can mean when, but they are used differently.

når:

Used for present, future, and repeated actions (whenever/when)
Also used in questions about time

Examples:

Når vi skrur på musikken, blir lyden høyere. – When(ever) we turn on the music, the sound gets louder.
Når kommer du? – When are you coming?

da:

Used only for one specific time in the past
Not used for general truths or repeated events

Example:

Da vi skrudde på musikken, ble lyden høyere. – When we turned on the music (that one time), the sound got louder.

In your original general statement, når is the correct choice. Da would sound like you are talking about one particular event in the past and would require past tense in the rest of the sentence.

Could I also say Lyden blir høyere når vi setter på musikk? Is that different?

Yes, that is both grammatical and natural. There are two changes:

setter på instead of skrur på
- sette på musikk = put some music on / start playing music
- Feels slightly more like starting playback (Spotify, a playlist, a record).
musikk (indefinite) instead of musikken (definite)
- sette på musikk – put on some music, in general
- sette på musikken – put on the (specific/known) music

Nuance:

Lyden blir høyere når vi skrur på musikken.
Suggests a specific system/music they regularly turn on.
Lyden blir høyere når vi setter på musikk.
More general: whenever we play music, the sound gets louder.

Why is it lyden blir høyere and not something like volumet blir høyere or lyden blir sterkere? Are those possible?

All of these are possible; they just focus on slightly different things.

Lyden blir høyere.
Most natural, everyday way to say The sound gets louder.
Volumet blir høyere.
More technical/specific: The volume gets higher.
Refers directly to the volume level (a slider, a knob).
Lyden blir sterkere.
Also possible: sterk can mean strong, and with sound it can mean loud.
This can sound a bit more physical/impactful: The sound becomes stronger / more powerful.

In normal conversation about music or devices, Lyden blir høyere is the standard expression for The sound gets louder.

What is the role of på in skrur på musikken? Can I just say skrur musikken?

In skru på, på is a small particle that is part of the verb phrase, similar to English particles in turn on, switch on.

å skru by itself = to screw/turn (physically rotate something)
å skru på = to turn on (a device, sound, etc.)

So:

Vi skrur på musikken. – We turn on the music.
Vi skrur musikken. – Sounds wrong/unnatural; it loses the meaning turn on.

You generally need på (or another particle like av, opp, ned) with skru when you talk about devices/sound:

skru på – turn on
skru av – turn off
skru opp – turn up
skru ned – turn down

Dropping på changes or destroys the intended meaning in this context.