Den nye færdighed bliver anvendt, når den svære sætning oversættes.

Because Danish normally uses a different definite pattern when an adjective comes before the noun.

• Without an adjective: færdigheden = the skill
• With an adjective: den nye færdighed = the new skill

So when you add nye, you usually use:

• den/det/de
  • adjective + noun

The noun itself is then usually in its basic singular/plural form, not with the attached definite ending.

The same pattern appears in den svære sætning.

In Danish, adjectives usually take -e in definite noun phrases.

So:

• en ny færdighed = a new skill
• den nye færdighed = the new skill

Because færdighed is part of a definite phrase introduced by den, the adjective becomes nye.

The same thing happens in:

• den svære sætning

Because both færdighed and sætning are common gender nouns in Danish.

Danish has two grammatical genders:

• common gender → uses en / den
• neuter → uses et / det

So:

• en færdighed → den færdighed
• en sætning → den sætning

If the noun were neuter, you would use det instead.

It forms the passive voice.

• anvende = to apply / use
• anvendt = past participle
• bliver anvendt = is being used / is used

So instead of saying who uses the skill, the sentence focuses on the skill itself.

Compare:

• Man anvender den nye færdighed = One uses the new skill
• Den nye færdighed bliver anvendt = The new skill is used

This is a very common way to make passives in Danish.

Danish has two common ways to make the passive:

1. blive + past participle

   • bliver anvendt
   • bliver oversat

2. -s passive

   • anvendes
   • oversættes

So oversættes means is translated / gets translated.

Your sentence mixes the two passive types:

• bliver anvendt
• oversættes

That is grammatically fine. Danish often does this, especially in formal or written language.

A fully parallel version could also be:

• Den nye færdighed anvendes, når den svære sætning oversættes. or
• Den nye færdighed bliver anvendt, når den svære sætning bliver oversat.

Yes, but it is often a matter of style more than meaning.

• bliver anvendt can sound a bit more dynamic or process-focused
• anvendes often sounds a bit more formal, written, or impersonal

In many contexts, both simply mean is used.

So:

• Den nye færdighed bliver anvendt
• Den nye færdighed anvendes

Both are natural, but the second often feels slightly more formal.

Når means when here, and it is used for something general, repeated, or expected.

This sentence is not about one specific past moment. It means something like:

• whenever this kind of translation happens
• when this sentence is translated in general

That is why når fits well.

A useful comparison:

• når = when, in general / repeated / future-related contexts
• da = when, for one specific event in the past
• hvis = if

So in this sentence, når is the natural choice.

Because når introduces a subordinate clause, and subordinate clauses do not use the normal main-clause verb-second pattern.

In a main clause, Danish often puts the finite verb in second position:

• Den svære sætning oversættes.

But after når, you get normal subordinate-clause order:

• når den svære sætning oversættes

So the subject den svære sætning comes before the verb oversættes.

Because Danish often uses the present tense for general truths, habits, instructions, and processes.

So:

• bliver anvendt
• oversættes

do not have to mean only right now. They can also describe what generally happens.

That is very similar to English:

• The new skill is used when the difficult sentence is translated.

It is a general statement, so present tense is natural.

The dictionary form is oversætte, meaning to translate.

Some related forms are:

• oversætte = infinitive, to translate
• oversætter = present active, translates / is translating
• oversættes = present passive, is translated
• oversat = past participle, translated

So in your sentence, oversættes is the present passive form.

It is acceptable, and many learners will see it often, but Danish comma rules allow two systems.

With start comma, you write:

• Den nye færdighed bliver anvendt, når den svære sætning oversættes.

Without start comma, you write:

• Den nye færdighed bliver anvendt når den svære sætning oversættes.

Both are accepted in modern Danish, depending on the comma style being followed. So the comma here is not wrong at all.

Yes, often it could.

• anvende = to apply/use
• bruge = to use

So:

• Den nye færdighed bliver anvendt
• Den nye færdighed bliver brugt

Both can work. However, anvende often sounds a bit more formal or technical than bruge. In educational, academic, or formal contexts, anvende is very common.

Because each noun phrase is separately definite:

• den nye færdighed
• den svære sætning

Each one means the ... something, so each one needs its own definite marker.

Danish does not let one den cover both nouns here, because they are in different noun phrases and play different roles in the sentence.