Hvis lækken kommer igen, må vi ringe til en håndværker med det samme.

Because the sentence starts with an if-clause: Hvis lækken kommer igen.

In Danish main clauses, the finite verb normally takes the second position. When the if-clause comes first, it counts as the first element, so the main clause has inversion:

Hvis lækken kommer igen, må vi ringe ...

Not:

Hvis lækken kommer igen, vi må ringe ...

So this is a very common Danish word-order pattern:

• Hvis ..., må vi ...
• Når ..., går jeg ...
• I morgen kommer han ...

Here hvis means if in a conditional sense.

Use hvis when something depends on a condition:

• Hvis det regner, bliver vi hjemme.

Use om more for whether/if in indirect questions:

• Jeg ved ikke, om det regner. = I don’t know whether it’s raining.

So in your sentence, hvis is the correct choice because it means if this happens, then...

Here må means must / have to / will have to.

So må vi ringe means something like:

• we must call
• we have to call
• we’ll have to call

It does not mean permission here. Danish må can sometimes mean may / be allowed to, but context decides the meaning.

In this sentence, the idea is necessity: if the leak comes back, calling a tradesperson becomes necessary.

Because må is a modal verb, and modal verbs in Danish are followed by a bare infinitive.

So you say:

• må ringe
• kan komme
• vil hjælpe
• skal arbejde

Not:

• må at ringe

This is similar to English:

• must call
• not must to call

Because lækken means the leak, not just a leak.

In Danish, definiteness is usually shown by adding an ending to the noun:

• en leak-type noun → -en for the ...
• here: lækken = the leak

The definite form suggests that both speaker and listener already know which leak is being talked about — for example, a leak in the house that has happened before.

That is a very natural thing to notice.

Danish often uses komme igen or komme tilbage for problems that return. So lækken kommer igen means:

• the leak comes back
• the leak returns

It is not a word-for-word match with the most natural English phrasing, but it is normal Danish.

Another possible Danish way to express the idea would be:

• Hvis det lækker igen ... = If it leaks again ...

So Danish can talk either about:

• the leak returning (lækken kommer igen) or
• it leaking again (det lækker igen)

Because Hvis lækken kommer igen is a subordinate clause, and it is followed by the main clause.

So the sentence is divided like this:

Hvis lækken kommer igen, / må vi ringe til en håndværker med det samme.

That comma helps show the structure:

• condition first
• result second

This is standard Danish punctuation.

Because the normal Danish expression for to call someone is ringe til nogen.

So:

• ringe til en håndværker = call a tradesperson

The preposition til is part of the usual pattern here.

Compare:

• Jeg ringer til lægen. = I’m calling the doctor.
• Kan du ringe til ham? = Can you call him?

Danish often uses fixed verb + preposition combinations, and this is one of them.

En håndværker means a tradesperson / skilled worker, such as a plumber, electrician, carpenter, and so on.

In this sentence, English might translate it as:

• a tradesperson
• a repair professional
• sometimes a handyman, depending on context

But håndværker is often broader and more professional than English handyman.

It is en håndværker because the noun has common gender and because the sentence means any suitable tradesperson, not a specific one.

Med det samme is a fixed expression meaning:

• immediately
• right away
• at once

You should learn it as a whole phrase, because the literal meaning does not help much.

So:

• må vi ringe til en håndværker med det samme means
• we must call a tradesperson immediately

A near synonym is straks.

Yes, Danish adverbs can often move, but the version in the sentence is very natural and neutral.

Current version:

• Hvis lækken kommer igen, må vi ringe til en håndværker med det samme.

You could also say:

• Hvis lækken kommer igen, må vi med det samme ringe til en håndværker.

That puts a bit more focus on immediately.

But kommer igen is the most natural order in the first clause, and med det samme at the end is a very normal place for that phrase.

That is a very good question, because both can relate to obligation.

In this sentence, må suggests necessity because of the situation:

• if the leak comes back, then calling someone becomes necessary

Skal often sounds more like:

• a plan
• an instruction
• a rule
• something scheduled

So:

• må vi ringe = we’ll have to call
• skal vi ringe could sound more like are we supposed to call / should we call / shall we call

That is why må fits especially well here: the recurring leak creates the need.