Policija može uvjeriti ubojicu.

Because policija is grammatically singular in Croatian. Unlike English police, which usually takes a plural verb, Croatian policija behaves like a singular noun meaning the police force. So the verb must also be singular:

• Policija može = The police can

If you meant individual police officers, you would normally use a plural noun such as policajci, and then the verb would be plural:

• Policajci mogu = The police officers can

Because after the modal verb moći = can / to be able to, Croatian normally uses the infinitive of the main verb:

• može uvjeriti = can convince

This is very common in Croatian:

• može doći = can come
• može vidjeti = can see
• može uvjeriti = can convince

Unlike English, Croatian does not use a separate word like to before the infinitive here.

In this sentence, može most naturally means can / is able to / has the possibility to.

So Policija može uvjeriti ubojicu most likely means that the police have the ability or a realistic chance to convince the murderer.

The verb moći can sometimes also express permission or general possibility, but that is less likely in this sentence.

Ubojicu is in the accusative singular, because it is the direct object of uvjeriti.

The basic dictionary form is ubojica. In this sentence, the police are doing the action, and the murderer is receiving the action, so Croatian uses the accusative:

• ubojica = murderer
• uvjeriti ubojicu = to convince the murderer

The ending changes from -a to -u in the accusative singular.

Because some Croatian masculine nouns, especially nouns for people, end in -a. They are still masculine even though the ending may look feminine to a beginner.

So ubojica is a masculine animate noun, and it behaves like other masculine nouns of this type, such as:

• kolega = colleague
• tata = dad
• vođa = leader

That is why you get forms like:

• taj ubojica = that murderer
• opasni ubojica = dangerous murderer
• vidim ubojicu = I see the murderer

No. They are a common aspect pair in Croatian.

• uvjeriti is perfective
  It focuses on the result: successfully convincing someone.
• uvjeravati is imperfective
  It focuses on the process, effort, repetition, or ongoing action of trying to convince someone.

So:

• Policija može uvjeriti ubojicu suggests the police can actually bring about the result.
• Policija može uvjeravati ubojicu would sound more like the police can try to persuade him or keep persuading him.

Yes. The sentence is grammatically complete.

Just as in English, Croatian can leave that part unstated if the context already makes it clear. For example, the full idea might be:

• Policija može uvjeriti ubojicu da prizna. = The police can convince the murderer to confess.
• Policija može uvjeriti ubojicu u nešto. = The police can convince the murderer of something.

But Policija može uvjeriti ubojicu. is still a complete sentence on its own.

Because Croatian does not have articles.

So nouns like policija and ubojica / ubojicu appear without words corresponding to English the or a/an. Whether something is definite or indefinite is usually understood from context.

That means the Croatian sentence can correspond to English in several ways depending on context, though one translation will usually sound most natural in the situation.

Yes. Croatian word order is more flexible than English word order because case endings show who is doing what.

The neutral order here is:

• Policija može uvjeriti ubojicu.

But you could also say:

• Ubojicu može uvjeriti policija.

That changes the emphasis, not the core meaning. This version puts more focus on ubojicu or on the contrast that it is the police who can convince him.

So word order in Croatian often helps show emphasis, topic, or contrast.

No. It is capitalized only because it is the first word of the sentence.

If it appeared in the middle of a sentence, it would normally be written policija, not Policija.

Normally no. Policija usually means the police as an institution, organization, or force.

If you want to talk about one police officer, you would use:

• policajac = male police officer
• policajka = female police officer

If you want to talk about several police officers:

• policajci = police officers

So in this sentence, policija is best understood as the police in a collective sense.