Klatreskoen er liten.

It is a compound noun:

• klatre = to climb
• sko = shoe
• klatresko = climbing shoe
• klatreskoen = the climbing shoe

So the whole sentence means The climbing shoe is small.

Because Norwegian usually writes compound nouns as a single word.

So:

• klatresko = climbing shoe
• not klatre sko

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

The ending -en marks the definite singular for many masculine nouns in Bokmål.

Here the base noun is:

• en sko = a shoe

Definite form:

• skoen = the shoe

Since klatresko ends with sko, it follows the same pattern:

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

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

So:

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

English uses a separate article, but Norwegian very often uses a suffix like -en, -a, or -et.

Because liten is the form used with a singular masculine/feminine noun here.

The noun sko is masculine, so you get:

• Skoen er liten = The shoe is small

Compare:

• et hus er lite = a house is small
  because hus is neuter

So the adjective changes to match the gender and number of the noun.

This is a very common question.

Here liten is a predicative adjective: it comes after the verb er and describes the subject.

• Klatreskoen er liten = The climbing shoe is small

In this position, the adjective agrees mainly with gender and number, not with definiteness in the same way as when it stands before the noun.

Compare:

• en liten klatresko = a small climbing shoe
• den lille klatreskoen = the small climbing shoe
• klatreskoen er liten = the climbing shoe is small

So lille is the form you expect before a definite noun, but after er, liten is correct.

Because this sentence uses er as a linking verb, just like English is.

Structure:

• Klatreskoen = subject
• er = is
• liten = adjective describing the subject

So it works very much like English:

• The climbing shoe is small

The adjective is not directly in front of the noun here; it is part of the predicate.

Yes. In modern Norwegian, verbs usually do not change by person.

So with å være (to be), you get:

• jeg er = I am
• du er = you are
• han/hun er = he/she is
• vi er = we are
• dere er = you are
• de er = they are

That is much simpler than English.

Grammatically, it means one shoe, because klatreskoen is singular.

If you wanted to talk about a pair or more than one, you would normally use the plural:

• Klatreskoene er små = The climbing shoes are small

So in your sentence, the grammar is singular.

The indefinite version would be:

• En klatresko er liten. = A climbing shoe is small.

Compare the two:

• En klatresko = a climbing shoe
• Klatreskoen = the climbing shoe

That is a useful pair to memorize.