Stemmeskruen endrer tonehøyden.

Because Norwegian usually writes compound nouns as a single word.

So:

• stemmeskrue = stemme
  • skrue
• tonehøyde = tone
  • høyde

This is very common in Norwegian. Where English often uses two words or a hyphen, Norwegian often prefers one compound word. The last part is the main part of the noun, so skrue tells you that stemmeskrue is a kind of screw/peg, and høyde tells you that tonehøyde is a kind of height—literally tone-height, meaning pitch.

Not in this sentence.

The word stemme can mean voice, but it is also related to the verb å stemme, which can mean to tune in a musical context. In stemmeskrue, it is the tuning idea that matters.

So stemmeskrue is best understood as tuning peg or tuning screw, not anything about a speaking voice.

In Norwegian, the definite article is usually attached to the end of the noun instead of being a separate word.

So:

• stemmeskrue = a tuning peg
• stemmeskruen = the tuning peg
• tonehøyde = pitch
• tonehøyden = the pitch

Because stemmeskrue and tonehøyde already end in -e, the definite form is made by adding -n:

• stemmeskrue → stemmeskruen
• tonehøyde → tonehøyden

So the sentence has two definite nouns: the tuning peg and the pitch.

Endrer is the present tense of å endre, which means to change.

A few useful points:

• å endre = infinitive, to change
• endrer = present tense, changes / is changing
• Norwegian does not change the verb for different persons the way English does.

So you get:

• jeg endrer
• du endrer
• han/hun endrer
• vi endrer

The form stays the same.

This is the normal word order for a simple main clause in Norwegian:

subject + finite verb + object

So here:

• Stemmeskruen = subject
• endrer = verb
• tonehøyden = object

This is straightforward, but it is also useful to know that Norwegian is a V2 language. That means the finite verb normally stays in the second position in main clauses.

For example:

• Stemmeskruen endrer tonehøyden.
• På gitaren endrer stemmeskruen tonehøyden.

In the second sentence, På gitaren comes first, so the verb endrer still stays second.

Because tonehøyde is more precise.

Tonehøyde specifically means pitch—how high or low a sound is.

Tonen is more general and can mean things like:

• a tone
• a note
• tone of voice
• manner or attitude in speech

So if you want to say that something changes the pitch, tonehøyden is the better word.

In Bokmål, this can be a little flexible.

The last part of the compound, skrue, determines the gender behavior of stemmeskrue. The noun skrue is often treated as feminine, but in Bokmål many feminine nouns can also take masculine-style forms.

So you may see:

• ei stemmeskrue / stemmeskrua
• en stemmeskrue / stemmeskruen

Your sentence uses stemmeskruen, which is completely normal in Bokmål.

Sometimes, yes, but the meaning shifts a little.

• endrer = changes
• forandrer = changes / alters
• justerer = adjusts

In this sentence, endrer tonehøyden is a very natural way to say that the tuning peg changes the pitch.

If you use justerer, that usually focuses more on the action of adjusting the peg itself, for example:

• Man justerer stemmeskruen. = You adjust the tuning peg.

So endrer is the clearest choice if the point is the effect on the pitch.

You would use en or ei with the noun instead of the definite ending.

So:

• en stemmeskrue or ei stemmeskrue = a tuning peg
• stemmeskruen = the tuning peg

A natural sentence would be:

• En stemmeskrue endrer tonehøyden.
• Ei stemmeskrue endrer tonehøyden.

If you want a slightly more general statement, you could also say:

• En stemmeskrue kan endre tonehøyden. = A tuning peg can change the pitch.

The dictionary forms are:

• en/ei stemmeskrue = a tuning peg
• å endre = to change
• en tonehøyde = a pitch

So the sentence is built from:

• definite subject: stemmeskruen
• present-tense verb: endrer
• definite object: tonehøyden