Barnet går inn i hulen.

Because barnet is the definite singular form: the child.

• et barn = a child
• barnet = the child

Barn is a neuter noun, and many neuter nouns add -et in the definite singular.

Går is the present tense of gå.

• å gå = to go / to walk
• går = goes / is going / walks / is walking

So in a normal present-tense statement, Norwegian uses går:

• Barnet går ... = The child goes / is going ...

Unlike English, Norwegian does not need a separate verb like is to make is going here.

It can mean either, depending on context.

Å gå literally has a strong connection to moving on foot, so walk is often a good translation. But in many everyday sentences, it is also translated more generally as go.

So:

• Barnet går inn i hulen could be understood as The child walks into the cave
• or simply The child goes into the cave

The exact English choice depends on how natural you want the translation to sound.

Because they do different jobs.

• inn = in / inward
• i = in / into

Together, inn i gives the idea of movement from outside to inside: into.

This is very common in Norwegian:

• gå inn i huset = go into the house
• løpe inn i rommet = run into the room

So inn adds the directional idea, and i introduces the place.

A very common difference is:

• i can describe being in something, or sometimes movement depending on context
• inn i clearly emphasizes movement into something

Compare:

• Barnet er i hulen = The child is in the cave
• Barnet går inn i hulen = The child goes into the cave

So inn i is especially useful when you want to make the movement inward very clear.

Hulen is the definite singular form of hule: the cave.

• en hule = a cave
• hulen = the cave

This noun is a common-gender noun in this form.

So the pattern is:

• indefinite singular: en hule
• definite singular: hulen

Yes, in many varieties of Bokmål, that is possible.

Some nouns can be treated as feminine as well as common gender in Bokmål:

• en hule → hulen
• ei hule → hula

Both can be acceptable depending on style, dialect, and personal preference.
So you may see both:

• Barnet går inn i hulen
• Barnet går inn i hula

They mean the same thing.

Because Norwegian usually puts definiteness at the end of the noun instead of using a separate word like English the.

So:

• the cave = hulen
• not something like the hule

This is one of the most important differences from English.

This is normal Norwegian main clause word order:

subject + verb + other elements

• Barnet = subject
• går = verb
• inn i hulen = adverbial/prepositional phrase

So it follows a very straightforward pattern:

• The child
  • goes
    • into the cave

Norwegian main clauses usually put the finite verb in the second position, and this sentence fits that pattern naturally.

Sometimes it might be understood, but it is less precise.

går i hulen could sound more like:

• walks around in the cave
• or simply goes in the cave

If your meaning is specifically enters the cave, then går inn i hulen is much better.

So for into the cave, use inn i.

Not in this sentence.

The natural order here is:

• går inn i hulen

You do sometimes see particles move around in Norwegian in other kinds of constructions, but with gå inn i + place, this order is the normal one.

So learners should treat gå inn i as a very common unit meaning go into.

A rough guide for English speakers:

• går has the letter å, which sounds somewhat like the vowel in British law or more, though not exactly.
• The r is pronounced, but how strong it sounds depends on dialect.

A very rough approximation is something like gor or gawr, but neither is perfect.

The important thing is that å is not like English a in cat or name.

No. Norwegian does not capitalize all nouns the way German does.

Barnet is capitalized only because it is the first word of the sentence.

In the middle of a sentence, it would normally be lowercase:

• Jeg ser barnet. = I see the child.

Yes. Norwegian present tense often covers several English present-time meanings.

So Barnet går inn i hulen can correspond to:

• The child goes into the cave
• The child is going into the cave
• The child walks into the cave
• The child is entering the cave

The best English translation depends on context.