Il poliziotto indaga sul caso.

Word by word:

• Il = the
• poliziotto = policeman / police officer (specifically masculine singular)
• indaga = investigates / is investigating
• sul = on / into / about the; here it is best understood as into the
• caso = case

So the whole sentence means The police officer is investigating the case or The policeman investigates the case, depending on context.

Italian usually uses an article where English often does too, and sometimes even where English might omit it.

Here, il poliziotto means the police officer. Without the article, poliziotto by itself would usually sound incomplete in a normal sentence.

So:

• Il poliziotto indaga sul caso = The police officer is investigating the case

Yes, poliziotto is grammatically masculine and normally refers to a male officer.

The feminine form is:

• la poliziotta = the policewoman / female police officer

If you want a more gender-neutral expression, Italian often uses:

• l’agente di polizia = the police officer

So this sentence specifically suggests a male officer unless the context says otherwise.

Indagare is the infinitive, meaning to investigate.

In a full sentence, you usually need a conjugated verb. Since the subject is il poliziotto = he / the officer, the verb must be in the third person singular of the present tense:

• indagare = to investigate
• io indago = I investigate
• tu indaghi = you investigate
• lui/lei indaga = he/she investigates

So indaga is the correct form for il poliziotto.

It can mean either one.

The Italian present tense often covers both:

• He investigates
• He is investigating

So Il poliziotto indaga sul caso could mean either:

• The police officer investigates the case
• The police officer is investigating the case

If you want to emphasize that the action is happening right now, Italian can also say:

• Il poliziotto sta indagando sul caso = The police officer is investigating the case

Because the verb indagare is commonly used with the preposition su when talking about what is being investigated.

So:

• indagare su qualcosa = to investigate something / to investigate into something

Then su + il contracts to sul:

• su + il = sul

So:

• sul caso = on the case / into the case

In this sentence, sul caso is the natural structure after indaga.

Sul is a contraction of:

• su
  • il = sul

This is very common in Italian. Prepositions often combine with articles.

A few similar examples:

• a + il = al
• di + il = del
• in + il = nel
• su + il = sul

So sul caso literally breaks down as on the case, though in natural English here we usually say into the case.

Here, caso means case, as in a police case or investigation.

In other contexts, caso can also mean things like:

• situation
• matter
• instance
• coincidence in some expressions

But in Il poliziotto indaga sul caso, the police context makes case the correct meaning.

This is the normal, straightforward word order in Italian:

• Subject + verb + complement
• Il poliziotto
  • indaga
    • sul caso

Italian word order is somewhat flexible, and parts of the sentence can move for emphasis, but this version is the most neutral and natural for a learner to use.

So this sentence is a good basic pattern to remember.

A helpful approximate pronunciation is:

• po-li-TSYOT-to

A few notes:

• the stress is on -ziot-
• the zz in poliziotto is pronounced like ts or tts
• the double tt is pronounced more strongly than a single t

A rough full pronunciation guide for the sentence is:

• eel po-li-TSYOT-to een-DA-ga sool KA-so

That is only approximate, but it can help you get started.

Yes. That is a very natural translation.

Depending on context, good English translations include:

• The policeman is investigating the case
• The police officer is investigating the case
• The policeman investigates the case

In modern English, police officer is often more neutral than policeman, but both can fit the Italian sentence if the officer is male.