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

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

Their basic forms are:

• en færdighed
• en sætning

When a singular common-gender noun is definite and has an adjective before it, Danish uses:

• den + adjective + noun

So:

• den nye færdighed
• den svære sætning

If the noun were neuter, you would use det instead, for example:

• det nye ord

Because Danish adjectives usually take -e in a definite phrase like this.

Here both noun phrases are definite:

• den nye færdighed
• den svære sætning

That gives the adjective the -e form:

• ny → nye
• svær → svære

This -e ending is very common after den, det, de.

Because when Danish uses den/det/de before an adjective, the noun usually stays in its basic singular/plural form, not the suffixed definite form.

So Danish says:

• den nye færdighed
• den svære sætning

not:

• den nye færdigheden
• den svære sætningen

This is a very important pattern in Danish:

• den gamle bil
• det store hus
• de røde bøger

Even though the whole phrase is definite, the noun itself does not take the definite ending here.

Bliver anvendt is a passive construction.

It is built from:

• bliver = present tense of blive
• anvendt = past participle of anvende

So the pattern is:

• blive + past participle

This is one common way to make the passive in Danish, especially when you want to focus on the action or process.

A similar example:

• Bogen bliver læst = the book is being read / the book is read

In your sentence, the focus is not on who applies the skill, but on the fact that it is applied.

Oversættes is also passive, but it is the -s passive instead of the blive-passive.

It comes from the verb oversætte and becomes passive by adding -s:

• oversætter = translates
• oversættes = is translated

So Danish has two common passive types:

• bliver oversat
• oversættes

Both can often mean roughly the same thing, but they can sound slightly different:

• blive-passive often feels a bit more like an event or process
• -s passive often feels more neutral, formal, or general

That means this sentence mixes two natural passive styles:

• bliver anvendt
• oversættes

That is perfectly normal Danish.

Når means when and is used for something that happens whenever, generally, or in a present/future sense.

That fits this sentence well: it describes what happens when that sentence is translated.

Compare:

• når = when / whenever
• hvis = if
• da = when in the past, or sometimes because

So:

• når den svære sætning oversættes = when the difficult sentence is translated

If you used hvis, the meaning would become more conditional:

• hvis den svære sætning oversættes = if the difficult sentence is translated

That is a different idea.

Because når introduces a subordinate clause, and subordinate clauses in Danish do not follow the main-clause verb-second pattern.

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

• Den nye færdighed bliver anvendt

But after a subordinating word like når, the clause normally has this order:

• når + subject + finite verb

So:

• når den svære sætning oversættes

not a main-clause style order with the verb immediately after når.

This is a very common Danish pattern after words like:

• når = when
• fordi = because
• at = that
• hvis = if

Yes. That is also correct.

Danish lets you place the subordinate clause first:

• Når den svære sætning oversættes, bliver den nye færdighed anvendt.

Or second:

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

The meaning is basically the same. The difference is mostly about focus and style.

Notice what happens when the når-clause comes first: the main clause still follows the Danish verb-second rule, so you get:

• ... bliver den nye færdighed anvendt

not:

• ... den nye færdighed bliver anvendt

You will often see a comma before a subordinate clause like the one introduced by når.

So this is very normal:

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

However, Danish comma practice has allowed more than one system, so you may also see sentences without that comma in some contexts.

For a learner, the safest practical point is:

• a comma before når in a sentence like this is very common and fully natural

So even if you later notice some variation, the comma here is not strange at all.