Policajac čuva ulaz.

Policajac is in the nominative singular. It is the subject of the sentence — the person doing the action.

• policajac = policeman / police officer
• nominative is the basic dictionary form here

So in Policajac čuva ulaz, policajac is the one who is guarding.

Yes — ulaz is in the accusative singular, but for many masculine inanimate nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: ulaz = entrance
• accusative: ulaz = entrance

That is normal in Serbian.

Compare that with a masculine animate noun like policajac:

• nominative: policajac
• accusative: policajca

So even though ulaz is the direct object, its form does not change.

Čuva is the 3rd person singular present tense form of the verb čuvati.

So:

• čuvati = to guard, to protect, to keep, to watch over
• čuva = he/she/it guards, is guarding

It agrees with the singular subject policajac.

A short present-tense pattern is:

• čuvam = I guard
• čuvaš = you guard
• čuva = he/she/it guards
• čuvamo = we guard
• čuvate = you plural/formal guard
• čuvaju = they guard

It can mean both, depending on context.

Serbian present tense often covers both:

• he guards
• he is guarding

So Policajac čuva ulaz could mean:

• The policeman guards the entrance
  or
• The policeman is guarding the entrance

The wider context tells you which is more natural.

Serbian does not have articles like English a / an / the.

So:

• policajac can mean a policeman or the policeman
• ulaz can mean an entrance or the entrance

Context tells you which meaning is intended.

That is why Policajac čuva ulaz can naturally be translated as The policeman is guarding the entrance, even though there is no separate word for the.

Yes. Serbian word order is fairly flexible because the cases help show what each noun is doing.

The neutral, straightforward order is:

• Policajac čuva ulaz.

But other orders are possible, for example:

• Ulaz čuva policajac.
• Čuva policajac ulaz.

These versions can sound more marked or emphasize different parts of the sentence.

Still, Policajac čuva ulaz is the most natural basic order for a simple statement.

The letter č is pronounced roughly like ch in church, but usually a bit firmer.

So:

• čuva sounds approximately like CHOO-va
• policajac is roughly po-lee-TSA-yats
• ulaz is roughly OO-laz

A few useful pronunciation notes:

• č and ć are different letters in Serbian, though English speakers often find them close at first.
• j is pronounced like English y
• c is pronounced like ts

So the ending of policajac is not like English -jac; it is closer to -yats.

Policajac is a masculine noun. If you want to refer specifically to a female police officer, Serbian often uses:

• policajka = female police officer

So you could say:

• Policajka čuva ulaz. = The policewoman / female police officer is guarding the entrance.

That said, in some contexts masculine forms can be used more generally, but policajka is the specifically feminine form.

Not exactly. Čuvati has a wider range of meanings than English guard.

It can mean:

• to guard
• to protect
• to watch over
• to look after
• to keep / store

Examples:

• Čuva dete. = He/she is looking after the child.
• Čuva psa. = He/she is watching the dog / taking care of the dog.
• Čuva novac. = He/she keeps the money safe.

In Policajac čuva ulaz, the meaning is something like guards or watches over.

Because Serbian usually does not need subject pronouns when the verb form already shows the person and number.

Here, čuva already tells you the subject is 3rd person singular. And the noun policajac is explicitly there anyway, so no pronoun is needed.

Using a pronoun like on would usually add emphasis:

• On čuva ulaz. = He is guarding the entrance.

But in the original sentence, just Policajac čuva ulaz is the normal way to say it.

Then it would normally change to the accusative singular:

• nominative: policajac
• accusative: policajca

This happens because policajac is masculine animate.

For example:

• Vidim policajca. = I see the policeman.

This is a useful contrast with ulaz, which is masculine inanimate and therefore keeps the same form in the accusative:

• Vidim ulaz. = I see the entrance.

The verb čuvati is imperfective.

That means it presents the action as:

• ongoing
• repeated
• habitual
• uncompleted

So Policajac čuva ulaz describes the guarding as an activity in progress or as a general fact.

A perfective partner can be used in some contexts, but for a simple present statement like this, čuvati is the natural choice. Perfective verbs generally do not normally describe an ongoing present action in Serbian the way English sometimes does.