Sutra moram ustati rano zbog sastanka u uredu.

Croatian often leaves out subject pronouns when they are already clear from the verb form.
Moram means I must / I have to, so ja is not necessary.

You could say Ja sutra moram ustati rano..., but that usually adds emphasis, like I have to get up early.

Because the time is already clear from sutra. Croatian often uses a present-tense form with a future time expression when the meaning is obvious.

So Sutra moram ustati rano naturally means Tomorrow I have to get up early.

A more explicitly future version is Sutra ću morati ustati rano, but that is a bit heavier and slightly more formal in feel.

This is a very common Croatian pattern:

morati + infinitive

So:

• moram = I must / I have to
• ustati = to get up

Together, moram ustati means I have to get up.

This is similar to English have to + verb.

This is about aspect, which is very important in Croatian verbs.

• ustati = perfective, a single completed act of getting up
• ustajati = imperfective, repeated or habitual getting up

In this sentence, the speaker means one specific act tomorrow, so ustati is the natural choice.

Compare:

• Sutra moram ustati rano. = I have to get up early tomorrow.
• Moram ustajati rano svaki dan. = I have to get up early every day.

Sutra is an adverb of time meaning tomorrow. It does not change for case, gender, or number.

So unlike many Croatian nouns, sutra stays the same.

Because rano is an adverb, not an adjective.

It describes the verb ustati:

• ustati rano = to get up early

If you were describing a noun, you would use an adjective instead. But here you are describing how someone gets up, so the adverb rano is correct.

Because the preposition zbog requires the genitive case.

The noun sastanak changes to sastanka in the genitive singular:

• nominative: sastanak
• genitive: sastanka

So:

• zbog sastanka = because of a/the meeting

Zbog and jer both relate to reason, but they are used differently.

• zbog
  • noun phrase = because of
• jer
  • full clause = because

So here:

• zbog sastanka u uredu = because of a meeting in the office

If you used jer, you would normally need a full clause, for example:

• jer imam sastanak u uredu = because I have a meeting in the office

Because u can take different cases depending on meaning:

• u + accusative = motion into somewhere
• u + locative = location in/at somewhere

Here the meeting is taking place in the office, so this is location, not movement. That is why Croatian uses u uredu.

Compare:

• Idem u ured. = I am going into the office.
• Sastanak je u uredu. = The meeting is in the office.

It can mean either. Croatian does not have articles like a and the.

So zbog sastanka could mean:

• because of a meeting
• because of the meeting

The exact meaning depends on context.

Yes. Croatian word order is more flexible than English word order.

The original sentence is natural and neutral:

• Sutra moram ustati rano zbog sastanka u uredu.

But you could also say:

• Moram sutra rano ustati zbog sastanka u uredu.
• Zbog sastanka u uredu sutra moram ustati rano.

The main meaning stays the same, but the emphasis changes. Putting sutra first highlights the time right away, which is very common.