Kad se vratim u općinu na šalter, službenica stavlja pečat na kopiju i provjerava dokumentaciju.

In Croatian, after kad (when) you very often use the present tense to talk about a future action, especially with a perfective verb like vratiti se.

• Kad se vratim… = When I return… (future)
• Past would be: Kad sam se vratio… = When I returned…
• If you want “whenever / every time,” you’d more likely use an imperfective verb: Kad se vraćam… = Whenever I’m coming back / When I come back (habitually)

Se marks a reflexive verb here: vratiti se = to return / to come back (literally “to return oneself”).

• vratiti = to return something (e.g., vratiti knjigu = return a book)
• vratiti se = to return (yourself), come back

So Kad se vratim is the normal way to say When I come back / return.

It’s mainly aspect:

• vratim (se) (from vratiti se) is perfective: focuses on completion (return and arrive back).
• vraćam (se) (from vraćati se) is imperfective: focuses on process or repetition (be returning / return regularly).

In this sentence, Kad se vratim… implies “once I’ve returned (and arrived back), then…”

Because the sentence starts with a subordinate time clause introduced by Kad. Croatian normally uses a comma to separate that clause from the main clause:

• Kad …, službenica …

If you reverse the order, the comma is usually not used:

• Službenica stavlja pečat … kad se vratim …

They describe two different “targets”:

• u općinu = into/to the municipality office (the institution/building)
• na šalter = to the counter/desk (the specific service point inside)

So it’s like: When I return to the municipal office, to the counter…

With u:

• u + accusative = movement/direction (to/into)
• u + locative = location (in/inside)

Here it’s direction: vratim u općinu = return to the municipality office → accusative (općinu). If it were “when I’m in the municipality office,” it would be u općini.

Same rule with na:

• na + accusative = movement to/onto (to the counter)
• na + locative = being at/on (at the counter)

Here it’s movement/goal: vratim … na šalter → accusative. If it were “at the counter,” it would be na šalteru.

• pečat is accusative (direct object) because you “put/stamp” something: stavlja (što?) pečat.
• na kopiju is also accusative because na
  • accusative expresses placing onto something (direction/goal).

So the structure is: staviti + object (acc) + na + target (acc).

Literally it’s puts/places a stamp, but idiomatically it means stamps:

• stavlja pečat na kopiju = she stamps the copy / she puts a stamp on the copy

Croatian often expresses “to stamp” this way, though pečatiti also exists.

službenica means a female clerk / female civil servant / female official.

• Male: službenik
• Female: službenica

It’s commonly used for someone working at an office counter in government/administration.

općina is a municipality (an administrative unit) and can also refer to the municipal office/building where you handle paperwork. In everyday speech, u općinu often means to the municipal office.

dokumentaciju is accusative singular of dokumentacija (a collective noun meaning documentation/paperwork). It behaves like a singular noun grammatically:

• provjerava (što?) dokumentaciju = checks the documentation/paperwork

If you meant individual documents, you could say dokumente (accusative plural).