Dragen skjuler seg i skogen.

Because dragen is the definite singular form of drage.

• en drage = a dragon
• dragen = the dragon

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

So the sentence uses dragen because it is talking about the dragon, not just any dragon.

Norwegian usually builds the into the noun itself.

For many common-gender nouns in Bokmål:

• en drage → dragen
• en skog → skogen

So:

• dragen already means the dragon
• skogen already means the forest

A separate word like den is usually added only in certain situations, especially when there is an adjective:

• dragen = the dragon
• den store dragen = the big dragon

Seg is a reflexive pronoun. It often means himself / herself / itself / themselves, depending on the subject.

Here it goes with the verb skjuler:

• å skjule noe = to hide/conceal something
• å skjule seg = to hide oneself or simply to hide

In natural English, we usually just say hide, but Norwegian often keeps the reflexive pronoun.

So skjuler seg is the normal way to say hides in this sense.

Yes. Skjuler is the present tense form of å skjule (seg).

A few useful forms:

• å skjule seg = to hide
• skjuler seg = hides / is hiding
• skjulte seg = hid / was hiding
• har skjult seg = has hidden / has been hiding

In this sentence, Dragen skjuler seg i skogen is a present-tense statement.

Because skogen means the forest.

Compare:

• skog = dictionary form, not normally used by itself in a sentence like this
• en skog = a forest
• skogen = the forest

So:

• i en skog = in a forest
• i skogen = in the forest

In your sentence, the speaker is referring to a specific or understood forest, so the definite form skogen is used.

The sentence has a very normal Norwegian main-clause order:

• Dragen = subject
• skjuler = finite verb
• seg = reflexive pronoun
• i skogen = prepositional phrase

So the pattern is:

Subject + verb + reflexive pronoun + place

That gives:

Dragen skjuler seg i skogen.

Norwegian main clauses usually follow the V2 rule, which means the finite verb comes in the second position. If you move i skogen to the front, the verb still stays second:

I skogen skjuler dragen seg.

That is also grammatical.

Both are possible, but they can feel slightly different.

• skjuler seg = hides / conceals itself, often a bit more formal, literary, or dramatic
• gjemmer seg = hides, often more everyday and common in speech

So:

• Dragen skjuler seg i skogen sounds fine, and a bit more story-like or descriptive.
• Dragen gjemmer seg i skogen might sound more conversational and everyday.

If this is from a fantasy or narrative context, skjuler seg fits very well.

In Bokmål, both drage and skog are usually common-gender nouns.

That means their usual pattern is:

• en drage → dragen
• en skog → skogen

For common-gender nouns, the definite singular ending is often -en.

Very generally, Norwegian noun patterns often look like this:

• common gender: en ... / ...en
• neuter: et ... / ...et

So the -en ending here tells you these nouns are in the definite singular.

A rough English-friendly guide is:

• skjuler ≈ SHYOO-ler or SHOO-ler
• seg ≈ sai or sayg, depending on accent

A few important points:

• skj is usually pronounced something like English sh
• the u in skjuler is not exactly the English oo sound; it is a more rounded Norwegian vowel
• the pronunciation of seg varies quite a lot by dialect and speaking style

So a rough full-sentence approximation might be:

DRAH-gen SHOO-ler sai i SKOO-gen

That is only approximate, but it is a useful starting point.

It is Bokmål.

A few clues are:

• skjuler as the present-tense form
• seg
• skogen

A Nynorsk version might look different, depending on style and word choice.

So if you are learning standard written Norwegian from beginner materials, this sentence is in Bokmål.