Policija čuva ulaz u zgradu.

Because policija is grammatically singular in Croatian. It is a collective noun meaning the police, but it behaves like a singular feminine noun.

So:

• policija čuva = the police guard / is guarding
• not policija čuvaju

This is different from English, where police usually takes a plural verb: The police are guarding...

Ulaz is in the accusative singular because it is the direct object of čuva.

The verb čuvati means to guard / protect / watch over, and the thing being guarded goes in the accusative.

So:

• čuva ulaz = guards the entrance

For this noun, the nominative and accusative singular look the same:

• nominative: ulaz
• accusative: ulaz

That is normal for many inanimate masculine nouns in Croatian.

Because u can take different cases depending on meaning:

• u + accusative = movement toward / into
• u + locative = location in / inside

Here, ulaz u zgradu literally means entrance into the building or entrance to the building, so Croatian uses u + accusative:

• u zgradu = into/to the building

Compare:

• ulaz u zgradu = entrance to the building
• u zgradi = in the building

So u zgradi would not fit this phrase.

Because Croatian often expresses this idea with a directional phrase:

• ulaz u zgradu = literally entrance into the building

This is a very natural Croatian way to say the entrance to the building.

Another possible structure would be:

• ulaz zgrade = the building's entrance / entrance of the building

But ulaz u zgradu is extremely common and sounds very natural.

It can mean any of those depending on context.

The verb čuvati is imperfective, so in the present tense it can describe:

• a current action: is guarding
• a habitual action: guards
• a more general meaning: protects / watches over

So Policija čuva ulaz u zgradu could mean:

• The police are guarding the entrance to the building
• The police guard the entrance to the building

Context tells you which one is meant.

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

So Croatian simply says:

• Policija čuva ulaz u zgradu.

Depending on context, that could correspond to:

• The police are guarding the entrance to the building
• Police are guarding an entrance to a building

Usually the situation makes it clear whether something is definite or indefinite.

Yes. Croatian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Policija čuva ulaz u zgradu.

But other orders are possible, for example:

• Ulaz u zgradu čuva policija.
• Čuva policija ulaz u zgradu.

These alternatives change the focus or emphasis, not the basic meaning.

For example:

• Ulaz u zgradu čuva policija puts more focus on the entrance to the building
• Policija at the start sounds like a neutral statement about what the police are doing

Policija is feminine singular, and yes, that matters because adjectives and verbs in some forms must agree with it.

In this sentence, the present-tense verb form čuva does not visibly show gender, but gender becomes more obvious in other structures, for example:

• hrvatska policija = Croatian police
• policija je bila prisutna = the police were present
  (with feminine singular agreement)

So even though the meaning is collective, grammatically policija is treated as a singular feminine noun.

A rough guide:

• č sounds like ch in church
• ž sounds like the s in measure
• c sounds like ts in cats

So:

• čuva ≈ CHOO-vah
• zgradu ≈ ZGRAH-doo
• policija ≈ po-lee-TSEE-ya
• ulaz ≈ OO-lahz

A full rough pronunciation of the sentence:

• po-lee-TSEE-ya CHOO-vah OO-lahz oo ZGRAH-doo

Yes, but the meaning and grammar would change slightly.

• Policija čuva ulaz u zgradu.
  = The police are guarding the entrance to the building.
  This refers to the police as an institution or force.

• Policajci čuvaju ulaz u zgradu.
  = Police officers are guarding the entrance to the building.
  This refers to individual officers, and the verb is plural: čuvaju.

So both are possible, but they are not exactly the same in nuance.