Først når stikket sidder ordentligt i stikkontakten, virker forlængerledningen som den skal.

Not literally. In this kind of sentence, først når means only when, not until, or only once.

So:

• Først når stikket sidder ordentligt i stikkontakten ... = Only when the plug is properly in the socket ... = It is not until the plug is properly in the socket that ...

This is a very common Danish pattern.

Examples:

• Først når du øver dig, bliver du bedre.
  Only when you practise do you get better.
• Først når han kommer hjem, spiser vi.
  We won’t eat until he gets home.

So først here does not mean first in a sequence. It marks a limit: something only happens after a certain condition is met.

This is because Danish is a V2 language: in a main clause, the finite verb usually comes in the second position.

The sentence begins with the subordinate clause:

• Først når stikket sidder ordentligt i stikkontakten

After that, the main clause starts, and the finite verb must come before the subject:

• virker forlængerledningen som den skal

So the structure is:

• [fronted clause] + [verb] + [subject]

That is why you get:

• ..., virker forlængerledningen ...

and not:

• ..., forlængerledningen virker ...

Compare:

• I dag arbejder jeg hjemme.
• Når jeg har tid, arbejder jeg hjemme.

In both cases, something comes first, so the verb comes before the subject in the main clause.

Danish often uses verbs like sidde, stå, and ligge where English would simply use be.

Here, stikket sidder i stikkontakten means something like:

• the plug is sitting in the socket
• more naturally: the plug is in the socket / inserted in the socket

Using sidder suggests that the plug is placed/fitted/seated in position.

This is very natural in Danish for things that are attached, inserted, or positioned.

Examples:

• Nøglen sidder i låsen.
  The key is in the lock.
• Billedet sidder på væggen.
  The picture is on the wall.
• Skruen sidder løst.
  The screw is loose.

So sidder is not about a person sitting here. It is about something being fixed or positioned in a certain way.

Here ordentligt means properly, correctly, or securely.

So:

• stikket sidder ordentligt i stikkontakten = the plug is properly/securely in the socket

It suggests that the plug must be fully inserted and making good contact.

More generally, ordentlig / ordentligt can mean:

• proper
• decent
• tidy
• correct

Examples:

• Gør det ordentligt.
  Do it properly.
• Et ordentligt måltid.
  A proper meal.
• Værelset er ikke ordentligt ryddet op.
  The room hasn’t been tidied properly.

These are related but different words.

• et stik = a plug
• stikket = the plug
  This is the definite form of et stik.
• en stikkontakt = a socket / wall outlet
• stikkontakten = the socket / the wall outlet

So in the sentence:

• stikket = the plug
• stikkontakten = the socket it goes into

Also note that stikkontakt is a compound noun:

• stik
  • kontakt

Danish makes lots of compound nouns this way.

Because Danish often uses the definite form when talking about a specific item that is understood from the situation.

Here we are talking about:

• the plug
• the socket
• the extension cord

Even without earlier context, the situation makes them identifiable: it is the plug belonging to the cord, the socket it goes into, and the extension cord being discussed.

Danish uses a suffix for definiteness:

• et stik → stikket
• en stikkontakt → stikkontakten
• en forlængerledning → forlængerledningen

This is one of the most important features of Danish nouns.

Because the plug goes in the socket, and Danish naturally uses i for that.

• i stikkontakten = in the socket

Even though English often says in the socket too, learners sometimes wonder because Danish can use different position verbs and prepositions than English. Here i is the normal choice.

You may also hear:

• sætte stikket i stikkontakten
  put the plug into the socket

Once it is there, Danish can say:

• stikket sidder i stikkontakten
  the plug is in the socket

So i is exactly what you should expect here.

Som den skal is a very common Danish expression meaning:

• as it should
• the way it is supposed to
• properly

So:

• virker forlængerledningen som den skal = the extension cord works as it should = the extension cord works properly

This phrase is extremely useful in everyday Danish.

Examples:

• Bilen kører som den skal.
  The car runs as it should.
• Computeren virker ikke som den skal.
  The computer isn’t working as it should.
• Alt fungerer som det skal.
  Everything works as it should.

Notice that English often uses properly where Danish prefers som den/det skal.

Because den refers back to forlængerledningen.

• en forlængerledning is a common-gender noun
• common-gender nouns are referred to with den

So:

• forlængerledningen ... som den skal

If the noun were a neuter noun (et-word), you would use det instead.

Examples:

• Et ur virker som det skal.
  A clock works as it should.
• En maskine virker som den skal.
  A machine works as it should.

So the pronoun agrees with the gender of the noun it refers to.

Yes. Danish very often forms nouns by joining words together into one compound.

• forlængerledning = extension cord
• literally something like extension-line/cable

This is completely normal in Danish.

Other examples:

• stikkontakt = socket
• fjernbetjening = remote control
• køleskab = refrigerator
• toothbrush-type compounds are especially common in Danish

English sometimes writes these as two words, hyphenates them, or uses one word depending on the expression. Danish strongly prefers a single compound word.

It is a subordinate clause, and here it expresses a mix of time and condition.

On its own, når often means when:

• Når jeg kommer hjem, laver jeg mad.
  When I get home, I make food / I’ll cook.

But with først når, the meaning becomes strongly conditional:

• only when
• not until

So in this sentence, the idea is not just time, but also a required condition:

• the extension cord works properly only if/when the plug is correctly inserted

That is why English might translate it with either:

• Only when ...
• Not until ...

depending on the style.

Yes. Danish allows other versions, but they shift the focus slightly.

For example:

• Forlængerledningen virker først som den skal, når stikket sidder ordentligt i stikkontakten.

This is also natural. It starts with forlængerledningen and puts the først later.

The original sentence:

• Først når stikket sidder ordentligt i stikkontakten, virker forlængerledningen som den skal.

puts strong emphasis on the condition first:

• Only when the plug is properly in the socket ...

So both can work, but the original highlights the condition more dramatically.

In Danish, virke can mean several things depending on context, including:

• work / function
• seem / appear
• have an effect

Here it clearly means work / function.

So:

• forlængerledningen virker = the extension cord works

Examples of other meanings:

• Han virker træt.
  He seems tired.
• Medicinen virker.
  The medicine works / has an effect.

So learners should know that virke is flexible, but in this sentence it is about functioning correctly.