Vi skifter sykkelslangen ettersom punkteringen er ved ventilen.

Sykkelslangen is the definite singular form, meaning the bicycle inner tube.

In Norwegian, definiteness is usually added as an ending:

• en sykkelslange = a bicycle inner tube
• sykkelslangen = the bicycle inner tube

Here, the speaker is talking about a specific tube, the one on the bicycle they are dealing with, so the definite form is natural.

It is a compound noun plus a definite ending:

• sykkel = bicycle
• slange = tube / hose
• sykkelslange = bicycle tube / inner tube
• sykkelslangen = the bicycle tube / inner tube

This is very common in Norwegian: nouns are often joined into one word.

That -n marks the noun as definite singular for many masculine nouns.

For example:

• en slange = a tube / hose
• slangen = the tube / hose

Since sykkelslange is treated as a masculine noun here, the definite form becomes sykkelslangen.

Punkteringen means the puncture. The sentence is referring to a specific puncture, namely the one they have found on the tube.

Compare:

• en punktering = a puncture / a flat
• punkteringen = the puncture / the flat

So both sykkelslangen and punkteringen are definite because both refer to specific, known things in the situation.

Ettersom means since, because, or as in the sense of giving a reason.

So:

• Vi skifter sykkelslangen ettersom punkteringen er ved ventilen. means:
• We’re replacing the inner tube because the puncture is by the valve.

In everyday Norwegian, fordi is often more common for because, but ettersom is perfectly correct and can sound slightly more formal or explanatory.

Because ettersom introduces a subordinate clause, and in Norwegian subordinate clauses the word order is normally subject + verb.

So:

• ettersom punkteringen er ved ventilen

Here:

• punkteringen = subject
• er = verb

That is normal subordinate-clause word order.

Then the subordinate clause comes first, and the main clause usually has inversion:

• Ettersom punkteringen er ved ventilen, skifter vi sykkelslangen.

Notice the main clause becomes:

• skifter vi not
• vi skifter

That is because Norwegian main clauses follow the V2 rule: the finite verb comes in the second position.

Norwegian often uses the present tense for actions happening now, for planned actions, or for what English might express as we’re replacing.

So:

• Vi skifter sykkelslangen can mean
  • We replace the inner tube
  • We are replacing the inner tube
  • We’re going to replace the inner tube depending on context.

This is very normal in Norwegian.

Yes, often you could.

• skifte = change / replace
• bytte = change / swap / replace

In this sentence, both would sound natural to many speakers:

• Vi skifter sykkelslangen
• Vi bytter sykkelslangen

Skifte can sound slightly more like replace/change, while bytte often feels a bit more like swap/change out, but in everyday usage there is a lot of overlap.

Ved means by, near, or at. Here it suggests the puncture is at the area of the valve, not literally on top of it in a simple surface sense.

So:

• ved ventilen = by the valve / at the valve

You may sometimes hear other prepositions in similar contexts, but ved is a natural choice when talking about a puncture located in the valve area.

Ventilen means the valve.

Again, this is the definite form:

• en ventil = a valve
• ventilen = the valve

In a bicycle context, this is the air valve on the inner tube.

It specifically says sykkelslangen, which is the inner tube, not the outer tire.

That distinction is useful:

• dekket = the tire
• sykkelslangen = the inner tube

So the sentence means they are replacing the inner tube because the puncture is near the valve, which is a place where repairs may be difficult or unreliable.

Yes, but in natural English We’re replacing the inner tube is usually better.

That is because:

• sykkelslange in a bike-repair context usually means inner tube
• change the tube is understandable in English, but replace the inner tube is often the clearest translation

So your translation is not wrong, but replace the inner tube is usually the most idiomatic choice.