Den voksne hjælper barnet i haven.

Because voksen is normally an adjective meaning adult or grown-up, but Danish can also use adjectives as nouns.

So:

• en voksen = an adult
• den voksne = the adult

In this sentence, voksne is not describing another noun like person. It is standing on its own and means the adult.

After a definite article like den, adjectives usually take the -e ending.

Compare:

• en voksen = an adult
• den voksne = the adult

So the -e here does not mean plural. It is the normal definite form of the adjective.

Den is used with common gender words, while det is used with neuter words.

In den voksne, the expression is treated like a common-gender human noun, so Danish uses den.

Very roughly:

• den = common gender singular definite
• det = neuter singular definite

Danish usually puts the definite article on the end of the noun.

So:

• et barn = a child
• barnet = the child

That is the normal definite form of the noun.

You often only get a separate article like det when there is an adjective:

• det lille barn = the small child

Because two different patterns are involved:

1. A normal noun by itself usually takes the definite ending:

   • barn → barnet

2. An adjective used with definiteness takes a separate article plus the -e form:

   • voksen → den voksne

So den voksne follows the adjective pattern, while barnet follows the ordinary noun pattern.

Hjælper is the present tense of at hjælpe.

So:

• at hjælpe = to help
• hjælper = help / helps

Danish verbs do not change according to person the way English does. The same present-tense form is used with all subjects:

• jeg hjælper
• du hjælper
• han hjælper
• vi hjælper

In this sentence, word order tells you:

• Den voksne = subject
• hjælper = verb
• barnet = object

So the adult is doing the helping, and the child is receiving the help.

If you switch the two noun phrases, the meaning changes:

• Barnet hjælper den voksne = The child helps the adult

Danish nouns do not usually show subject/object with different endings, so word order is very important.

I haven is a prepositional phrase showing location.

• i = in
• haven = the garden

So it tells you where the helping happens.

Also notice:

• en have = a garden
• haven = the garden

Because haven is the definite form, meaning the garden.

Compare:

• en have = a garden
• haven = the garden

So i haven means in the garden, not just in garden.

This sentence is quite close to English word order:

• subject → verb → object → place

So:

• Den voksne = subject
• hjælper = verb
• barnet = object
• i haven = place

But Danish main clauses follow the V2 rule, which means the finite verb must be in the second position. If you move another element to the front, the verb still stays second:

• I haven hjælper den voksne barnet.

That still means the same thing, but with extra emphasis on in the garden.

Not in this sentence.

Even though -e can sometimes appear in plural forms, here den shows that it is singular:

• den voksne = the adult singular
• de voksne = the adults plural

So den is the clue that this is one adult, not several.

A learner often notices hjælper first.

In hjælper, the h is not pronounced like a clear English h. The beginning sounds closer to y in yes than to a strong hj cluster in English spelling.

A few other useful points:

• voksne has a consonant cluster that may feel compact to English speakers.
• barnet ends with a weak final syllable.
• haven also has a weak second syllable.

So the sentence may look straightforward in writing, but the spoken rhythm is softer and more reduced than an English speaker might expect.