Ako krenemo sada, stići ćemo u Zagreb prije šest sati.

Because Croatian normally uses the present tense after ako for real future conditions, just like English says If we leave now, not If we will leave now.

So:

• Ako krenemo sada = If we leave/set off now
• stići ćemo... = we will arrive...

Here krenemo is the present form of a perfective verb, so it naturally refers to a single future action.

Krenemo comes from krenuti, which means to start off, to set out, to leave, to get going.

In this sentence, the most natural translation is:

• If we leave now
• or If we set off now

So it is not just general movement; it means the moment of departure.

This is a question of aspect, which is very important in Croatian.

• krenuti → perfective: to set off once, as a completed event
• kretati → imperfective: to be setting off, to set off repeatedly, or to describe the action more generally

In this sentence, we are talking about one specific departure, so krenemo is the natural choice.

• Ako krenemo sada... = If we set off now...
• Ako krećemo sada... would sound less natural here and suggests something more ongoing or arranged.

Stići ćemo is the future tense.

It is made from:

• stići = infinitive, to arrive / to reach
• ćemo = we will

So:

• stići ćemo = we will arrive

This is the standard Croatian future construction.

Because ćemo is a clitic, and clitics usually do not stand at the beginning of a clause.

Croatian likes clitics in the second position of the clause. That is why stići ćemo is normal.

You can also say:

• Mi ćemo stići... = We will arrive...

But starting the clause with ćemo by itself is not the usual pattern.

Here stići means to arrive or to reach a destination.

So:

• stići ćemo u Zagreb = we will arrive in Zagreb

Depending on context, stići can also mean things like to manage, to catch up, or to get somewhere in time, but in this sentence it clearly means arrive.

Because Croatian changes the case depending on whether you mean movement toward a place or location in a place.

• u Zagreb = to Zagreb → destination, so accusative
• u Zagrebu = in Zagreb → location, so locative

Compare:

• Idemo u Zagreb. = We are going to Zagreb.
• Mi smo u Zagrebu. = We are in Zagreb.

In this sentence, it means before six o’clock.

So the whole sentence means:

• If we leave now, we’ll arrive in Zagreb before six o’clock.

A useful note: out of context, prije šest sati can also mean six hours ago, so context matters. Here, because the sentence is about arrival time, the meaning is clearly before six.

Because Croatian uses different forms of sat depending on the number.

• jedan sat = one hour
• dva / tri / četiri sata = two / three / four hours
• pet / šest / sedam sati = five / six / seven hours

So:

• šest sati is the correct form after šest

The same pattern is used when telling time:

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

No. Croatian often leaves subject pronouns out because the verb ending already shows who the subject is.

• krenemo already tells you it means we leave/set off
• ćemo already tells you it means we will

So Ako krenemo sada, stići ćemo... is completely normal without mi.

If you add mi, it usually gives emphasis:

• Ako mi krenemo sada... = If we leave now...

Yes to both.

Croatian word order is fairly flexible, so these are all natural:

• Ako krenemo sada...
• Ako sada krenemo...

Both mean the same thing.

Also:

• sada = now
• sad = shorter, more conversational form of sada

So Ako sad krenemo... is very common in everyday speech.

Because the sentence starts with a conditional subordinate clause:

• Ako krenemo sada = if we leave now

Then comes the main clause:

• stići ćemo u Zagreb prije šest sati = we’ll arrive in Zagreb before six o’clock

In Croatian, it is standard to separate that introductory ako-clause with a comma.

A rough English-friendly guide:

• Ako ≈ AH-ko
• krenemo ≈ KREH-neh-mo
• stići ćemo ≈ STEE-ći CHEH-mo
• Zagreb ≈ ZAH-greb
• prije ≈ PREE-yeh
• šest ≈ shest
• sati ≈ SAH-tee

A few useful sound notes:

• j is like English y
• š is like sh
• ž is like the s in measure
• ć is a soft sound, somewhat like a softer version of ch

You do not need to make it perfect immediately; being consistent and clear matters most.