Klatreskoen er hverken stor eller tung.

Klatreskoen is a compound noun:

• klatre = to climb
• sko = shoe
• -en = the definite ending, meaning the

So klatreskoen literally means the climbing shoe.

Norwegian very often joins nouns together into one word, where English would often use two words. So:

• klatresko = climbing shoe
• klatreskoen = the climbing shoe

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

So:

• en klatresko = a climbing shoe
• klatreskoen = the climbing shoe

That -en is the definite ending for a masculine noun in Bokmål.

This is one of the biggest differences from English. English uses a separate word, the, but Norwegian often adds definiteness directly to the noun.

Because the noun is already definite through the ending -en.

So Norwegian usually says:

• klatreskoen = the climbing shoe

not:

• den klatreskoen in this kind of simple sentence

You can use den with a definite noun, but usually when there is an adjective:

• den store klatreskoen = the big climbing shoe

Without an adjective, klatreskoen by itself is enough.

Hverken ... eller means neither ... nor.

In this sentence:

• hverken stor eller tung = neither big nor heavy

It connects two things that are both being denied.

A useful pattern is:

• X er hverken A eller B = X is neither A nor B

Examples:

• Han er hverken trøtt eller sulten. = He is neither tired nor hungry.
• Boken er hverken lang eller vanskelig. = The book is neither long nor difficult.

Usually, if you want the meaning neither ... nor, hverken ... eller is the natural choice.

So:

• Klatreskoen er hverken stor eller tung. = natural

If you say:

• Klatreskoen er ikke stor eller tung

that can sound more ambiguous. It may be understood, but it is not as clear or elegant for neither/nor meaning.

So for learners, a good rule is:

• Use hverken ... eller for neither ... nor

Because they come after er and describe the noun predicatively.

In Norwegian, adjectives used after verbs like er are often in the basic dictionary form, but they still have to agree when necessary. Here the noun is singular and common gender, so the forms are:

• stor
• tung

Compare:

• Skoen er stor. = The shoe is big.
• Huset er stort. = The house is big.
  Here stort is used because hus is neuter.
• Skoene er store. = The shoes are big.
  Here store is used because the noun is plural.

So the form depends on the noun being described.

Because the noun is singular.

Klatreskoen refers to one specific shoe, so singular adjective forms are used.

For a singular common-gender noun, the usual forms are:

• stor
• tung

If it were plural, you would get:

• Klatreskoene er hverken store eller tunge. = The climbing shoes are neither big nor heavy.

So:

• singular: stor, tung
• plural: store, tunge

Er is the present tense of å være = to be.

So:

• er = is / are

In this sentence, it links the subject klatreskoen to the descriptions stor and tung:

• Klatreskoen er ... = The climbing shoe is ...

This is exactly the same kind of job that is does in English.

This is normal Norwegian main-clause word order:

• subject
  • verb
    • the rest

So:

• Klatreskoen = subject
• er = verb
• hverken stor eller tung = complement

Norwegian main clauses usually put the finite verb in the second position. Here the subject comes first, so the verb er comes right after it.

The indefinite singular form is:

• en klatresko = a climbing shoe

The forms are:

• en klatresko = a climbing shoe
• klatreskoen = the climbing shoe

And in plural:

• klatreskoer = climbing shoes
• klatreskoene = the climbing shoes

This is useful because Norwegian learners often need to connect the definite form they see in a sentence with the basic dictionary form.

In Bokmål, sko is normally a common-gender noun, and in practice you will often learn it with en:

• en sko
• skoen

That is why you get klatreskoen.

More broadly, many learners are taught to think of this as the masculine/common pattern:

• en in the indefinite
• -en in the definite

So here, the ending fits the noun class of sko.

Yes, it sounds completely natural.

It is a normal descriptive sentence, and hverken ... eller is a standard and natural way to say neither ... nor.

A Norwegian speaker would easily understand it as a straightforward statement about the climbing shoe’s size and weight.