Sukkeret er sødt på frugten.

Because sukkeret is the definite form, meaning the sugar rather than just sugar in general.

In Danish, definiteness is often added as an ending instead of a separate word like English the:

• sukker = sugar
• sukkeret = the sugar

Even though sugar is a mass noun, Danish can still make it definite when it refers to specific sugar, such as the sugar on a particular piece of fruit.

Danish usually puts the at the end of the noun as a suffix.

So:

• frugt = fruit

• frugten = the fruit

• sukker = sugar
• sukkeret = the sugar

This is one of the biggest differences from English. Instead of saying something like the fruit, Danish often says frugten.

Because the adjective must agree with the noun, and sukker is a neuter noun.

In Danish, a predicate adjective after er still agrees with the noun:

• en frugt er sød = a fruit is sweet
• et æble er sødt = an apple is sweet
• sukkeret er sødt = the sugar is sweet

So the -t in sødt shows that the noun is singular neuter.

You learn it from the dictionary form: et sukker in theory, though with mass nouns you will more often just see sukker or sukkeret in actual use.

Danish nouns belong mainly to two genders:

• common gender: takes en
• neuter: takes et

Since sukker is neuter, its definite form is sukkeret, and its adjective form here is sødt.

Frugten means the fruit, so it refers to a specific fruit.

Compare:

• på en frugt = on a fruit
• på frugten = on the fruit

The definite form is used when the fruit is already known from the context, visible, or understood by speaker and listener.

På usually means on or onto.

So på frugten suggests that the sugar is on the fruit, probably on its surface.

That is different from:

• i frugten = in the fruit

So if the idea is sugar located inside the fruit, Danish would more naturally use i. If the idea is sugar coating or sitting on the fruit, på fits well.

Yes. This is a very normal Danish main-clause pattern:

• Sukkeret = subject
• er = verb
• sødt = subject complement / adjective
• på frugten = prepositional phrase

So the structure is basically:

Subject + verb + complement + prepositional phrase

That is very similar to English word order here.

It is grammatically correct, but how natural it sounds depends on the situation.

It sounds most natural if you are talking about actual sugar that is physically on some fruit, for example sugar sprinkled on fruit.

If you mean that fruit contains sweet sugar, Danish might more naturally say something like:

• Sukkeret i frugten er sødt = the sugar in the fruit is sweet
• Frugten er sød = the fruit is sweet

So the given sentence works best when på frugten really means on the fruit.

Usually no. Sukkeret normally means a specific, identifiable sugar.

If you want to talk about sugar as a substance in general, Danish more often uses just sukker:

• Sukker er sødt = sugar is sweet

So:

• Sukker er sødt = general statement
• Sukkeret er sødt = specific sugar

Frugt is a common-gender noun.

Its basic forms are:

• en frugt = a fruit
• frugten = the fruit

This is why the definite ending is -en, not -et.

So in this sentence you have:

• sukkeret = definite neuter noun
• frugten = definite common-gender noun

That difference is completely normal in Danish.