Sutra moram produžiti osobnu iskaznicu u općini.

Croatian word order is fairly flexible because case endings show grammatical roles. Sutra is placed first to set the time frame (“tomorrow”) as the topic. You could also say Moram sutra produžiti osobnu iskaznicu u općini, which is still natural—just a slightly different emphasis.

Moram means I must / I have to. The base form (dictionary form) is morati = “to have to/must.”
Present tense (useful to recognize):

• moram (I must)
• moraš (you must)
• mora (he/she/it must)
• moramo (we must)
• morate (you plural/formal must)
• moraju (they must)

Yes—produžiti is typically perfective, focusing on completing the action: “to renew/extend (successfully).”
The common imperfective counterpart is produživati, which emphasizes the process or repeated/habitual action.
So in this sentence, perfective makes sense because you’re talking about a single completed task you need to do tomorrow.

Literally it can mean “extend,” but with documents (ID cards, passports, permits) it’s commonly used in the sense of renew / extend the validity. In everyday terms, it’s the normal verb you’d use for renewing an ID.

Osobnu iskaznicu is in the accusative singular because it’s the direct object of produžiti (“to renew what?”).

• Nominative: osobna iskaznica
• Accusative: osobnu iskaznicu

Both words change because the adjective must agree with the noun in gender, number, and case.

Iskaznica is feminine, which you can usually guess because it ends in -a in the nominative singular. That’s why the adjective is osobna (feminine nominative) and becomes osobnu in the accusative.

In Croatia, osobna iskaznica is the standard term for a national ID card. In casual speech, people often shorten it to osobna (“ID”), e.g. Moram produžiti osobnu. Context matters, but it’s very common.

U općini uses u + locative to mean “in/at (a place)” when talking about location.
Općini is the locative singular of općina (“municipality/municipal office”).

• Nominative: općina
• Locative: u općini

In this context it usually means at the municipal office (the institution/building where you handle paperwork), not just “somewhere within the municipality.” Croatian often uses the same word for both, and the bureaucratic context makes the intended meaning clear.

People do sometimes say na općini in informal speech, but the more standard/neutral option is u općini. Use u općini if you want to sound safely correct.

No—Croatian usually omits subject pronouns because the verb ending already shows the person. Moram clearly means “I must,” so ja is only added for emphasis or contrast: Ja moram... (“I (specifically) have to...”)

After morati, Croatian normally uses the infinitive: moram produžiti. That’s the standard pattern: modal-ish verb + infinitive.

Both are correct. The difference is mainly emphasis and flow:

• Sutra moram... highlights “tomorrow” as the key info.
• Moram sutra... highlights the obligation (“I have to...”), then adds when.

Negation is formed by negating moram:

• Sutra ne moram produžiti osobnu iskaznicu u općini. = “Tomorrow I don’t have to renew the ID card at the municipal office.”

Ne goes directly before the verb: ne moram.

Yes. Another very common choice is obnoviti (“to renew”), especially in more formal contexts:

• Sutra moram obnoviti osobnu iskaznicu u općini.
  Both are widely understood; produžiti strongly suggests “extend validity,” which fits many document-renewal situations.