Sutra mi počinje jutarnja smjena, pa moram ustati prije šest.

Because sutra is a time adverb meaning tomorrow. In Croatian, words like danas (today), sutra (tomorrow), and jučer (yesterday) usually stand on their own, without a preposition.

So:

• Sutra = tomorrow
• Sutra mi počinje jutarnja smjena = Tomorrow my morning shift starts

Mi is the unstressed dative form of ja and literally means to me.

In sentences like this, Croatian often uses a dative pronoun where English would use my. So:

• Sutra mi počinje jutarnja smjena
  literally: Tomorrow to me starts the morning shift
• natural English: My morning shift starts tomorrow

This is very natural Croatian. You could also say moja jutarnja smjena, but that would make my more explicit.

Because mi is a clitic: a short, unstressed word that usually goes in second position in the clause.

So in:

• Sutra mi počinje...

the first element is Sutra, and the clitic mi comes right after it.

This is a very common Croatian pattern. Clitics do not usually go wherever you want; they tend to follow this special placement rule.

Croatian often uses the present tense for future events when the time is already clear from a word like sutra.

So počinje here is present in form, but it refers to a future event:

• Sutra mi počinje jutarnja smjena = My morning shift starts tomorrow

This is especially common for scheduled or expected events.

A future form is sometimes possible in other contexts, but here the present sounds very natural.

Because jutarnja smjena is the subject of the verb počinje, so it is in the nominative case.

• smjena is a feminine singular noun
• jutarnja is the adjective and agrees with it

So:

• jutarnja smjena = nominative singular
• jutarnju smjenu would be accusative singular, which is not needed here

It means morning shift.

• jutarnja = morning
• smjena = shift

In work-related contexts, smjena is the normal word for a work shift, like an early shift, night shift, etc.

Here pa means something like so, and so, or therefore.

It connects the two ideas like this:

• Tomorrow my morning shift starts, so I have to get up before six.

It is a very common everyday connector in Croatian, often sounding more natural and conversational than a more formal word for therefore.

Because moram means I must / I have to, and after morati Croatian normally uses an infinitive.

So:

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

Together:

• moram ustati = I have to get up

You do not use a second finite verb here.

Because Croatian very often uses the present form moram to express a real, understood obligation, even when the action itself is in the future.

So moram ustati here means:

• I have to get up
• in context: I’ll have to get up

Since sutra already sets the time frame, the sentence is clear.

A more explicitly future wording can exist in some contexts, but moram ustati is very natural here.

This is about aspect.

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

Here the speaker means one specific act on one specific morning, so ustati is the normal choice.

Because with clock times, Croatian often omits sati when it is obvious.

So:

• prije šest = before six
• prije šest sati = before six o’clock

Both are understood, but the shorter version is very common in normal speech.

Also, prije means before.

Because Croatian often drops subject pronouns when the verb ending already shows who the subject is.

• moram already means I must
• so ja is not necessary

If you say ja moram, that adds emphasis, like I have to.

Yes. Croatian word order is fairly flexible.

For example, you could also say:

• Jutarnja smjena mi počinje sutra.

That is still grammatical. The original sentence:

• Sutra mi počinje jutarnja smjena...

puts sutra first, which makes tomorrow the starting point of the message.

So the order can change, but the original sounds very natural.

Because the sentence has two clauses:

• Sutra mi počinje jutarnja smjena
• pa moram ustati prije šest

In standard writing, Croatian normally uses a comma before pa when it links clauses like this. The comma helps show the pause and the relationship between the two ideas.