Larmet var på när tjuven bröt sig in.

Because larmet is the definite singular form of ett larm.

• ett larm = an alarm
• larmet = the alarm

The noun larm is an ett-word in Swedish, so the definite ending is -et.

In this sentence, the speaker is talking about a specific alarm, so Swedish uses larmet.

Here, var på means was on, was activated, or was armed.

So:

• Larmet var på = The alarm was on

This is a very natural Swedish way to talk about devices or systems being switched on. With an alarm, it often means the system was armed, not necessarily that it was making noise.

Not usually. It normally means the alarm system was on/armed.

If you want to say the alarm actually started making noise, Swedish would more naturally say something like:

• Larmet gick igång = The alarm went off
• Larmet gick = The alarm went off
• Larmet tjöt = The alarm wailed/sounded

So in your sentence, the idea is usually that the alarm was activated before or during the break-in, not necessarily that it was already ringing.

Tjuven is the definite singular form of en tjuv.

• en tjuv = a thief
• tjuven = the thief

Here, Swedish presents the thief as a specific person in the situation: the thief.

That said, En tjuv bröt sig in would also be possible in another context if you were introducing the thief as a thief for the first time. So this is partly a matter of context and perspective.

Bröt sig in means broke in or forced entry.

It comes from the verb bryta sig in, which is an idiomatic expression used when someone enters a place illegally by force.

A rough literal breakdown is:

• bröt = broke
• sig = oneself
• in = in

But you should not translate it word for word. The whole expression bryta sig in simply means to break in.

Because bryta sig in is a reflexive verb expression in Swedish.

The sig is part of the standard pattern. It does not mean the thief literally broke himself. It is just how Swedish forms this expression.

Compare:

• bryta = break
• bryta sig in = break in

So when talking about burglary or forced entry, Swedish normally uses sig.

Bröt is the past tense (preterite) of bryta.

Principal forms:

• bryta = infinitive, to break
• bröt = past tense, broke
• brutit = supine/past participle form used in compounds, as in har brutit

So:

• tjuven bröt sig in = the thief broke in

In Swedish, with this kind of reflexive verb expression, the normal order is:

verb + reflexive pronoun + particle

So:

• bröt sig in

not

• bröt in sig

This is the usual pattern in many similar expressions, for example:

• ta sig in = get in
• ge sig av = set off / leave
• ställa sig upp = stand up

So bröt sig in is the standard word order.

När means when here and introduces a subordinate clause:

• när tjuven bröt sig in = when the thief broke in

The full sentence has:

• main clause: Larmet var på
• subordinate clause: när tjuven bröt sig in

So the structure is:

[Main clause] + [when-clause]

Because after när, Swedish uses a subordinate clause, and subordinate clauses normally keep the order:

subject + verb

So:

• när tjuven bröt sig in

with:

• tjuven = subject
• bröt = finite verb

This is different from main clauses, where Swedish often uses verb-second word order.

For example, if you start the whole sentence with the när clause, you get:

• När tjuven bröt sig in, var larmet på.

Here, the main clause changes word order to var larmet på because the sentence begins with another element.

Yes. That is completely correct.

Both of these are natural:

• Larmet var på när tjuven bröt sig in.
• När tjuven bröt sig in, var larmet på.

The meaning is basically the same. The difference is emphasis:

• starting with Larmet var på focuses first on the alarm
• starting with När tjuven bröt sig in focuses first on the time/event

Also notice the word order in the second version:

• När tjuven bröt sig in, var larmet på
• not När tjuven bröt sig in, larmet var på

That is because Swedish main clauses follow the verb-second rule.