Barnet slår på bordet, når det vil have mere juice.

Because -et is the definite article attached to the noun.

• barn = a child
• barnet = the child

Danish usually puts the at the end of the noun instead of using a separate word like English does.

Since barn is a neuter noun (et barn), the definite form becomes barnet.

For the same reason as barnet.

• et bord = a table
• bordet = the table

So both barnet and bordet are definite forms of et-nouns.

This is a very common question.

In Danish, pronouns often follow the grammatical gender of the noun, not the natural gender you might expect in English.

• barn is a neuter noun
• so it can be referred to as det

So:

• Barnet ... når det vil have mere juice
  literally uses det because barn is grammatically neuter.

However, in real life, if the child is known and the speaker is thinking of the child as a boy or girl, Danish can also use:

• han = he
• hun = she

So det is grammatically normal here, especially in a general statement.

Slår på means something like hits/bangs on.

• slå = to hit / strike
• slå på bordet = hit/bang on the table

The preposition på helps give the idea of striking on a surface.

In this sentence, it suggests the child is banging on the table to show it wants more juice.

Because Danish uses på for something happening on a surface.

• på bordet = on the table

So slår på bordet is literally hits on the table, which in natural English becomes hits/bangs on the table.

Because når det vil have mere juice is a subordinate clause.

In Danish main clauses, the finite verb is usually in second position. But in subordinate clauses, the word order is different and more like English:

• det vil have = it wants
• not vil det have

So:

• Barnet slår på bordet, når det vil have mere juice.

If the når-clause came first, the whole sentence would be:

• Når det vil have mere juice, slår barnet på bordet.

Notice that the main clause after the initial subordinate clause then has inversion:

• slår barnet
• not barnet slår

That is a very typical Danish pattern.

Here it can mean when in a repeated, general sense, which often comes close to whenever in English.

So the sentence means something like:

• The child bangs on the table when/whenever it wants more juice.

Because the sentence describes a habitual action, English could use either translation depending on context.

Yes, literally vil have is wants to have, but in Danish it is a very normal way to express wanting something.

• jeg vil have kaffe = I want coffee
• barnet vil have mere juice = the child wants more juice

So even though the words look a bit different from natural English structure, this is standard Danish.

Because juice here is being treated like an uncountable noun, just like in English.

• mere juice = more juice

You do not need an article before it.

Compare:

• mere vand = more water
• mere mælk = more milk
• mere juice = more juice

Because Danish often uses the present tense for things that happen regularly, typically, or as a general pattern.

So:

• Barnet slår på bordet, når det vil have mere juice

means this is what the child does in that situation.

English works the same way:

• The child bangs on the table when it wants more juice.

This is a habitual meaning, not necessarily something happening only right now.

It breaks down like this:

• Barnet = subject
• slår = verb
• på bordet = prepositional phrase
• når det vil have mere juice = subordinate clause telling when

So the pattern is:

• [main clause], [subordinate clause]

More specifically:

• Barnet = the child
• slår på bordet = bangs on the table
• når det vil have mere juice = when it wants more juice

In modern Danish, comma use before subordinate clauses can vary depending on the comma system being followed.

So you may see both:

• Barnet slår på bordet, når det vil have mere juice.
• Barnet slår på bordet når det vil have mere juice.

Both can occur, though many learners will often meet the version with the comma.

So yes, the comma is normal, but you may also see sentences without it.

It would be:

• Når det vil have mere juice, slår barnet på bordet.

This shows an important Danish rule: when something other than the subject comes first, the finite verb still stays in second position in the main clause.

So:

• slår barnet
• not barnet slår

That inversion is one of the key word-order patterns in Danish.