Reservehjulet har et sommerdekk.

Because Norwegian usually writes compound nouns as a single word.

• reserve = reserve / spare
• hjul = wheel
• reservehjul = spare wheel
• reservehjulet = the spare wheel

This is very common in Norwegian. English often uses two words where Norwegian uses one.

The ending -et is the definite form of a neuter noun.

For hjul:

• et hjul = a wheel
• hjulet = the wheel

The same pattern applies to the compound:

• et reservehjul = a spare wheel
• reservehjulet = the spare wheel

A useful rule: in a compound noun, the last part usually determines the gender. Since hjul is neuter, reservehjul is neuter too.

Because dekk is also a neuter noun.

So the forms are:

• et dekk = a tire
• dekket = the tire

And with the compound:

• et sommerdekk = a summer tire
• sommerdekket = the summer tire

So both reservehjul and sommerdekk are neuter nouns.

Har means has.

So the sentence structure is:

• Reservehjulet = the spare wheel
• har = has
• et sommerdekk = a summer tire

Norwegian uses har here because the spare wheel is being described as having or being fitted with a summer tire.

If you used er, that would mean is, which would express identity or description, not possession.

This is an important vocabulary point:

• hjul = wheel
• dekk = tire

So reservehjul literally means spare wheel, while sommerdekk means summer tire.

In everyday English, people sometimes say spare tire, but Norwegian often uses reservehjul, which is closer to spare wheel.

Yes. This is a very standard main-clause word order:

Subject + verb + object

• Reservehjulet = subject
• har = verb
• et sommerdekk = object

So it works just like a basic English sentence such as The spare wheel has a summer tire.

Yes, dekk can be tricky because the indefinite singular and indefinite plural often look the same.

For example:

• et dekk = a tire
• flere dekk = several tires

So sommerdekk could mean either summer tire or summer tires depending on context.

But in this sentence, it is clearly singular because of et:

• et sommerdekk = a summer tire

Then you would use the definite form of sommerdekk:

• et sommerdekk = a summer tire
• sommerdekket = the summer tire

So the sentence would become:

Reservehjulet har sommerdekket.

Grammatically that is possible, though the meaning may depend on context.

Usually, the gender of a compound noun comes from the last element.

Examples here:

• reservehjul ends with hjul, so it behaves like hjul
• sommerdekk ends with dekk, so it behaves like dekk

Since both hjul and dekk are neuter, both compounds take et in the indefinite singular.

In a yes/no question, Norwegian usually puts the verb first:

Har reservehjulet et sommerdekk?

That means Does the spare wheel have a summer tire?

So compared with the statement:

• Reservehjulet har et sommerdekk.

the verb har moves in front.

Yes, this is standard Bokmål.

A Nynorsk version would likely be:

Reservehjulet har eit sommardekk.

The main differences are:

• et in Bokmål becomes eit in Nynorsk
• sommer- often becomes sommar- in Nynorsk

But the sentence you were given is perfectly normal Bokmål.