Det er ikke bare vandhanen, men også lækken bag skabet, der skal repareres.

This is a very common Danish emphasis pattern: Det er ... der ...

It is called a cleft sentence. Danish uses it to highlight what the speaker wants to focus on. Here, the focus is on what needs repairing.

So instead of the more neutral:

Ikke bare vandhanen, men også lækken bag skabet skal repareres.

the speaker says:

Det er ikke bare vandhanen, men også lækken bag skabet, der skal repareres.

That gives something like It is not just the faucet, but also the leak behind the cabinet, that needs to be repaired.

Here der introduces a relative clause, and it refers back to the things being talked about.

In Danish, when the relative word is the subject of the relative clause, der is very common and often preferred.

In der skal repareres, the thing referred to is the thing that must be repaired, so it functions as the subject of that clause.

A rough breakdown is:

• Det er ... = it is ...
• der skal repareres = that must be repaired

English often uses that here. Danish usually uses der in this kind of subject relative clause.

This is the Danish equivalent of not just / not only ..., but also ...

So:

• ikke bare vandhanen = not just the faucet
• men også lækken bag skabet = but also the leak behind the cabinet

Even though bare often means just, in this pattern ikke bare ... men også ... it works idiomatically as not only ... but also ...

You may also see:

ikke kun ..., men også ...

That means the same thing and is also very common.

Because Danish usually puts the definite article at the end of the noun instead of using a separate word like the.

So:

• en vandhane = a faucet/tap

• vandhanen = the faucet/tap

• en læk = a leak

• lækken = the leak

• et skab = a cabinet/cupboard
• skabet = the cabinet/cupboard

The endings depend on the noun’s gender:

• common gender nouns usually take -en
• neuter nouns usually take -et

So vandhane and læk are en-words, while skab is an et-word.

Bag is the preposition meaning behind.

So:

• bag skabet = behind the cabinet

This is the normal standard way to say it before a noun phrase.

You may also hear bagved, but bagved is often more adverb-like, for example:

• Han står bagved. = He is standing behind.

Before a noun, bag skabet is the most straightforward choice.

There are two important things going on here:

1. skal is a modal verb
2. repareres is a passive infinitive

After a modal verb in Danish, you normally use the infinitive without at.

So:

• skal reparere = must repair
• skal repareres = must be repaired

The -s form here is a passive form. Danish often uses this instead of a separate be + past participle structure.

So skal repareres is a very natural Danish way to say must be repaired.

Not mainly. In this sentence, skal expresses necessity, obligation, or what needs to happen.

So here it is closer to:

• must be repaired
• needs to be repaired
• has to be repaired

It is not just a simple future marker.

Danish skal can sometimes point to the future, but in a sentence like this, the sense is clearly required action, not merely future time.

Yes, lækken is a normal word.

The base noun is en læk, meaning a leak, especially in practical contexts like pipes, roofs, plumbing, or water damage.

So:

• en læk = a leak
• lækken = the leak

You could also say lækagen from en lækage. That is also correct, and can sound a bit more formal or technical depending on the context.

So both are possible:

• lækken bag skabet
• lækagen bag skabet

Yes. A few alternatives are possible.

A more direct version:

Ikke bare vandhanen, men også lækken bag skabet skal repareres.

A very neutral version:

Både vandhanen og lækken bag skabet skal repareres.

The original with Det er ... der ... is more emphatic. It draws special attention to the items that need repair.

So the difference is mostly one of focus and style, not basic meaning.

There are two commas here, and they help show the structure.

1. The comma after vandhanen
This separates the first part of the contrast from the men også part:

• ikke bare vandhanen, men også lækken bag skabet

2. The comma before der skal repareres
This marks the start of the relative clause:

• der skal repareres

In Danish punctuation, the comma before a subordinate or relative clause depends partly on which comma system is being used. So you may sometimes see this part written without that second comma in modern texts.

But with the commas shown, the structure is very clear.