Politiet undersøker innbruddet.

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

• politi = police / a police force
• politiet = the police

Here, -et is the definite ending for a neuter noun.

Grammatically, politiet is usually treated as a singular collective noun: it refers to the police as one institution or body.

That can feel strange for English speakers, because English the police normally takes a plural verb. In Norwegian, politiet is the normal way to say the police in this kind of sentence.

Also, Norwegian present-tense verbs do not change for singular vs. plural, so undersøker would look the same anyway.

For the same reason: it is the definite form of the noun.

• innbrudd = a burglary / a break-in
• innbruddet = the burglary / the break-in

So both nouns in the sentence are definite:

• politiet = the police
• innbruddet = the burglary

In Norwegian, the definite article is often attached to the noun as a suffix.

So instead of:

• the police
• the burglary

you get:

• politiet
• innbruddet

A separate definite word can appear too, but usually when there is an adjective:

• innbruddet = the burglary
• det alvorlige innbruddet = the serious burglary

So with no adjective, the suffix alone is enough.

Undersøker is the present tense of å undersøke.

• å undersøke = to investigate / examine / look into
• undersøker = investigates / is investigating

In this sentence, English would most naturally use is investigating or are investigating, but Norwegian just uses the ordinary present tense.

Because Norwegian usually uses the simple present for actions that are happening now.

So:

• Politiet undersøker innbruddet
  can mean:
• The police investigate the burglary
• The police are investigating the burglary

In context, the second one is usually the natural English translation.

If you really want to stress that the action is in progress, Norwegian can use expressions like holder på å:

• Politiet holder på å undersøke innbruddet
  = The police are in the middle of investigating the burglary

But that is not necessary in the original sentence.

It follows the normal subject–verb–object pattern:

• Politiet = subject
• undersøker = verb
• innbruddet = object

So literally, the structure is:

The police + investigate/are investigating + the burglary

This is a very standard Norwegian sentence pattern.

Yes. Å undersøke is a flexible verb and can mean:

• investigate
• examine
• inspect
• look into

In this sentence, because the object is innbruddet, the best English choice is usually investigate.

But in another context:

• Legen undersøker pasienten = The doctor examines the patient

So the exact translation depends on what is being examined or investigated.

Then you would use the indefinite form:

Politiet undersøker et innbrudd.

Here:

• et innbrudd = a burglary / a break-in

So the difference is:

• innbruddet = the burglary
• et innbrudd = a burglary

The original sentence uses the definite form, which suggests a specific burglary that is already known in the situation.

It can match both, depending on context.

• break-in is often a natural everyday translation
• burglary can sound a bit more formal or legal in English

So innbruddet could be translated as either the break-in or the burglary. The best choice depends on the tone of the translation.