Neprijatelj poznaje ubojicu.

Because ubojicu is the accusative singular form, and this noun is the direct object of the verb.

• ubojica = nominative singular, the dictionary form
• ubojicu = accusative singular

In this sentence, the enemy is doing the action, and the murderer is receiving it, so Croatian uses the accusative: ubojicu.

Yes. Ubojica is a masculine noun referring to a male person, even though its nominative singular ends in -a.

Croatian has a group of masculine nouns like this, especially for people, for example:

• sudac = judge
• vođa = leader
• ubojica = murderer / killer

So you should learn ubojica as a masculine noun with an unusual-looking nominative ending.

Because it is the subject of the sentence, so it is in the nominative singular.

The nominative is the basic form of the noun, and for neprijatelj, that basic form is simply neprijatelj.

If it were the direct object instead, it would change form because it is masculine animate:

• nominative: neprijatelj
• accusative: neprijatelja

So in this sentence, neprijatelj is the one doing the knowing.

Poznaje is the 3rd person singular present tense form of poznavati.

So it agrees with the singular subject neprijatelj:

• ja poznajem = I know
• ti poznaješ = you know
• on/ona poznaje = he/she knows

Here, neprijatelj is singular, so the verb is also singular: poznaje.

Because Croatian usually uses poznavati for knowing a person or being acquainted/familiar with someone.

A very useful distinction is:

• znati = to know a fact, to know how
• poznavati = to know a person, place, or thing through acquaintance/familiarity

So:

• Znam odgovor. = I know the answer.
• Poznajem Marka. = I know Mark.

In your sentence, the object is a person, ubojicu, so poznaje is the natural choice.

Croatian shows that mainly through case endings, not just word order.

In this sentence:

• neprijatelj = nominative subject
• ubojicu = accusative object

So the structure is:

• The enemy = subject
• knows
• the murderer = object

That is why the sentence cannot mean that the murderer knows the enemy, even though English relies more heavily on word order.

Yes. Croatian word order is more flexible than English because the cases already show who is subject and who is object.

The neutral order here is:

• Neprijatelj poznaje ubojicu.

But other orders are possible for emphasis, contrast, or style, for example:

• Ubojicu poznaje neprijatelj.
• Poznaje ubojicu neprijatelj.

These do not automatically change the basic meaning. They mainly change what is being highlighted.

Croatian has no articles like English the and a/an.

So neprijatelj can mean:

• the enemy
• an enemy

And ubojicu can mean:

• the murderer
• a murderer

Which one sounds best depends on the context. English must choose an article, but Croatian usually leaves that to the situation.

A few useful pronunciation points:

• j = y sound, like in yes
• c = ts, like in cats
• lj = a soft ly-like sound, roughly similar to the lli in million for many English speakers

Very roughly:

• Neprijatelj ≈ neh-pri-YA-tely
• poznaje ≈ poz-NA-ye
• ubojicu ≈ u-bo-YI-tsu

These are only rough English approximations, but they help you get close.