Fotstøtten brukes når sittestillingen endres.

Because fotstøtten is the definite singular form of fotstøtte.

• fotstøtte = a footrest
• fotstøtten = the footrest

The noun is a compound:

• fot = foot
• støtte = support

So literally it is something like foot-support, i.e. footrest.

In Norwegian instructions and manuals, the definite form is very common because it refers to the relevant part of the object being discussed: the footrest, the backrest, the button, etc.

This -s is the s-passive, a very common passive form in Norwegian.

• brukes = is used
• endres = is changed / changes

So:

• Fotstøtten brukes = The footrest is used
• sittestillingen endres = the sitting position is changed / the sitting position changes

This passive is especially common in:

• formal writing
• manuals
• instructions
• neutral descriptions

A more expanded alternative would be:

• Fotstøtten blir brukt når sittestillingen blir endret

But that sounds heavier and less natural in this kind of sentence.

It can feel a bit like either one, depending on context.

With verbs like endre, the -s form can be understood as:

• a true passive: is changed
• a more neutral process: changes

In a sentence like this, Norwegian often avoids saying exactly who changes the sitting position. The important point is simply that the sitting position is being altered.

So in practice, English translations might vary:

• The footrest is used when the sitting position is changed
• The footrest is used when the sitting position changes
• in more natural instructional English: Use the footrest when changing your sitting position

Når means when in the sense of:

• when / whenever
• a general situation
• something that happens under a certain condition

That fits this sentence, because it is describing a general rule or instruction.

Compare:

• når = when, whenever
• da = when, at that time, usually about a specific past event

So here når is the correct choice because the sentence is not about one particular past moment. It is saying what applies in general.

Because når introduces a subordinate clause, and subordinate clauses do not use normal Norwegian main-clause inversion.

In a main clause, Norwegian usually has verb-second word order:

• Sittestillingen endres.

But after a subordinating word like når, the subject usually comes before the verb:

• når sittestillingen endres

So this is standard subordinate-clause word order.

Sittestillingen is a compound noun:

• sitte = to sit
• stilling = position
• sittestilling = sitting position / seating posture
• sittestillingen = the sitting position

Norwegian forms compounds very freely and normally writes them as one word, where English often uses two words.

So:

• English: sitting position
• Norwegian: sittestilling

Then the definite ending is added:

• sittestilling → sittestillingen

This is very normal in Norwegian.

Even when a sentence expresses a general rule, Norwegian often uses the definite form for the relevant parts or functions of a known object. In a manual, the reader already knows which footrest and which sitting position are being talked about.

So the sentence naturally uses:

• fotstøtten = the footrest
• sittestillingen = the sitting position

This is similar to how English manuals often say things like:

• The handle is adjusted...
• The seat is moved...
• The button is pressed...

even though the statement is instructional and general.

Yes. Brukes is the present tense passive of bruke.

• å bruke = to use
• bruker = uses / use
• brukes = is used / are used

Norwegian often uses the present tense in instructions and general statements, just as English does:

• Døren åpnes forsiktig. = The door is opened carefully.
• Maskinen brukes daglig. = The machine is used daily.

So even though English needs is used, Norwegian can express that with a single present passive form: brukes.

Yes. A more active, user-directed version could be:

• Bruk fotstøtten når du endrer sittestillingen.

That means:

• Use the footrest when you change the sitting position

This version directly addresses the user with an implied you. The original sentence is more neutral and typical of manuals because it avoids mentioning the user directly.

So the difference is mainly tone:

• Fotstøtten brukes ... = impersonal, formal, instructional
• Bruk fotstøtten ... = direct instruction

The letter ø is a vowel that does not exist in standard English.

A rough guide:

• it is similar to ö in German
• or the vowel in French peu
• your lips are rounded, but the sound is made further forward in the mouth than o

In fotstøtten, you hear it in støt-.

A very rough English-friendly approximation might be something like STUR with rounded lips, but there is no exact English equivalent.

Also, the compound is typically stressed on the first part:

• FOT-støtten

with a secondary stress later in the word.