Indagano sul furto.

Indagano is the third-person plural, present indicative of indagare.

So it means they investigate or, depending on context, they are investigating.

Examples:

• Indagano sul caso. = They are investigating the case.
• I giornalisti indagano sui fatti. = The journalists investigate the facts.

Italian often leaves out subject pronouns because the verb ending already shows who is doing the action.

Here, -ano tells you the subject is they.

So:

• Indagano sul furto. and
• Loro indagano sul furto.

both mean the same thing, but the version without loro is more natural unless you want emphasis.

Because in Italian, su + il normally contracts to sul.

So:

• su + il = sul

This is very common with prepositions and articles:

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

So sul furto literally comes from su il furto, but the contracted form is the correct one.

A very common pattern is:

indagare su + something

It means to investigate into / to investigate / to look into something.

So:

• indagare sul furto
• indagare sul caso
• indagare sulle cause

This is just the usual Italian construction. English uses different patterns depending on the verb, so it is best to learn indagare su as a set phrase.

Italian uses the definite article more often than English does.

Here, il furto usually refers to a specific theft, one already known in the context.

So sul furto is like saying:

• about the theft
• into the theft

Without the article, furto would sound more general or abstract in many contexts.

It can mean either one, depending on context.

Italian present tense often covers both:

• a general/habitual meaning: they investigate
• a current ongoing meaning: they are investigating

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

• Stanno indagando sul furto.

But the simple present indagano is very common and natural.

Not exactly.

Furto means theft. It is the general word for stealing.

A few useful distinctions:

• furto = theft
• rapina = robbery
• borseggio = pickpocketing

So if an English speaker is thinking of robbery, furto is not always the right match. In this sentence, furto specifically means theft.

A simple pronunciation guide is:

een-dah-GAH-noh sool FOOR-toh

A few notes:

• indagano has the stress on ga
• su is pronounced soo
• furto has the stress on the first syllable: FUR-to

Also, the g in indagano is a hard g, like in go.

Usually, learners should stick with indagare su.

So the safest and most natural pattern is:

• indagare sul furto
• indagare sul caso

You may sometimes encounter more direct constructions in formal or journalistic language, but for everyday correct Italian, indagare su is the best choice to learn first.