Stölden måste polisanmälas idag.

Because stölden is the definite singular form of stöld.

• stöld = theft / a theft
• stölden = the theft

Swedish often marks definiteness by adding an ending to the noun, instead of using a separate word like the in English.

So:

• en stöld = a theft
• stölden = the theft

Since the sentence is talking about a specific theft, Swedish uses stölden.

Måste is a modal verb, like must, can, or should in English.

After a modal verb in Swedish, the next verb normally comes in the infinitive without att.

So:

• måste polisanmälas
• not måste att polisanmälas

Compare:

• Jag vill sova = I want to sleep
• Jag kan komma = I can come
• Det måste göras = It must be done

This is very similar to English, where we also say must report, not must to report.

The -s marks the passive voice.

The base verb is polisanmäla, which means to report to the police.
When it becomes polisanmälas, it means to be reported to the police.

So:

• polisanmäla = report to the police
• polisanmälas = be reported to the police

This -s passive is very common in Swedish.

A few examples:

• Boken säljs här. = The book is sold here.
• Det måste göras. = It must be done.
• Stölden måste polisanmälas idag. = The theft must be reported to the police today.

Both are possible ways to express a passive idea, but they are used a little differently.

In this sentence, polisanmälas is the natural, compact choice. It sounds direct and standard.

Compare:

• Stölden måste polisanmälas idag.
• Stölden måste bli polisanmäld idag.

The first one is more neutral and idiomatic here.
The second one is possible, but often feels a bit heavier or more focused on the resulting state/process.

As a learner, a good rule is:

• -s passive is very common in formal, neutral, and general statements
• bli + past participle is also passive, but often used when emphasizing that something gets done

Because this is a passive sentence.

In an active sentence, the object receives the action:

• Någon måste polisanmäla stölden idag.
• Someone must report the theft to the police today.

Here, stölden is the object.

In the passive version, that object becomes the grammatical subject:

• Stölden måste polisanmälas idag.

So stölden is the subject of the sentence grammatically, even though it is not performing the action.

This works much like English:

• Someone must report the theft today.
• The theft must be reported today.

Because that is a very normal place for a time expression in Swedish.

Swedish often puts adverbs like idag, imorgon, nu, and snart after the verb phrase:

• Jag kommer imorgon.
• Hon måste gå nu.
• Stölden måste polisanmälas idag.

But Swedish word order is flexible, and idag can be moved for emphasis:

• Idag måste stölden polisanmälas.

That gives more emphasis to today.

So the sentence-final position is neutral and natural, not the only possible option.

Because Swedish main clauses normally follow the verb-second rule, often called V2.

That means the finite verb must come in the second position in a statement.

Here:

1. Stölden
2. måste

So the sentence is correct as a normal statement:

• Stölden måste polisanmälas idag.

If you say:

• Måste stölden polisanmälas idag?

then it becomes a question: Must the theft be reported to the police today?

So the difference is mainly:

• Stölden måste ... = statement
• Måste stölden ...? = question

Yes. Måste is the present-tense form used here, and it expresses current necessity: the reporting needs to happen today.

A useful thing to know is that måste does not change for person:

• jag måste
• du måste
• han/hon måste
• vi måste

So it works a lot like English must in that way.

In this sentence, the focus is not really on present-time action, but on present obligation/necessity.

Yes. Swedish very often forms new words by combining parts into one compound word.

Here:

• polis = police
• anmäla = report

Together:

• polisanmäla = report to the police

English often uses several words where Swedish uses one compound word. This is extremely common in Swedish vocabulary.

So it is important not to split it into separate words in Swedish.

Not in this sentence if you mean a specific theft already known in the context.

• en stöld = a theft
• stölden = the theft

If the sentence is about one particular theft, stölden is the right form.

If you said:

• En stöld måste polisanmälas idag

that would sound more like a theft must be reported today, which is possible in some contexts, but it gives a different meaning. It sounds less specific.

Yes, it sounds fairly neutral-to-formal, mainly because of the verb polisanmäla, which is a standard administrative/legal word.

This is the kind of sentence you might see in:

• official communication
• insurance contexts
• police-related instructions
• written notices

It is not unnatural in everyday Swedish, but it does sound a bit more official than very casual conversation.

In casual speech, people might say something longer, such as explaining that someone needs to contact the police, but polisanmäla itself is still a normal everyday word.

A natural active version would be:

• Någon måste polisanmäla stölden idag.

That means someone must report the theft to the police today.

Here is the relationship:

• Active: Någon måste polisanmäla stölden idag.
• Passive: Stölden måste polisanmälas idag.

The passive is useful when the important thing is the theft, not who does the reporting.