Vlak za Zagreb polazi u sedam.

Croatian does not have articles like the or a/an.

So vlak can mean:

• the train
• a train

You understand which one is meant from context. In this sentence, English usually translates it as The train to Zagreb leaves at seven.

Here za Zagreb means to Zagreb or for Zagreb in the sense of destination.

With means of transport, Croatian often uses:

• vlak za Zagreb = the train to Zagreb
• autobus za Split = the bus to Split
• let za London = the flight to London

So za is very common when talking about a train, bus, or plane whose destination is a place.

Because Zagreb is a masculine inanimate noun, and in this case its accusative form is the same as its nominative form.

After za in this meaning, Croatian uses the accusative:

• za Zagreb

But for this noun, nominative and accusative look identical, so you do not see a change in form.

Polazi is the 3rd person singular present tense form of polaziti.

It agrees with vlak:

• vlak = singular
• polazi = it leaves / departs

So literally it is:

• Train for Zagreb leaves at seven.

In Croatian, the present tense is often used for scheduled future events, especially with timetables:

• Vlak polazi u sedam. = The train leaves at seven.

Because Croatian, like English, often uses the present tense for scheduled or fixed future events.

Compare:

• English: The train leaves at seven.
• Croatian: Vlak ... polazi u sedam.

This is especially common for:

• departures
• arrivals
• timetables
• opening times
• programmed events

So this is completely natural Croatian.

Croatian usually uses u with clock time:

• u jedan = at one
• u pet = at five
• u sedam = at seven

So u sedam is the normal way to say at seven.

In this expression, the number is in the accusative, but for sedam the form looks the same, so you just see sedam.

Yes. Both are correct:

• u sedam
• u sedam sati

The version with sati is a little more explicit, but in everyday speech people very often leave sati out when the meaning is already clear.

So:

• Vlak za Zagreb polazi u sedam.
• Vlak za Zagreb polazi u sedam sati.

Both are natural.

Sometimes, but polazi is more precise here.

• polazi = departs / sets off
• ide = goes

For trains, buses, flights, and timetables, polaziti is often the better verb when you mean the moment of departure.

So:

• Vlak za Zagreb polazi u sedam. = The train to Zagreb departs at seven.

If you used ide, it might still be understood, but polazi sounds more exact and more typical for departure information.

No, Croatian word order is fairly flexible.

The neutral order here is:

• Vlak za Zagreb polazi u sedam.

But you could also hear:

• U sedam polazi vlak za Zagreb.
• Vlak polazi u sedam za Zagreb.
  although this is less neutral and depends more on emphasis

Changing the order usually changes the focus or emphasis, not the basic meaning.

For example:

• U sedam polazi vlak za Zagreb. puts more attention on the time.

Croatian normally uses the noun itself when introducing or identifying what is acting:

• Vlak za Zagreb polazi u sedam.

A pronoun like on = he/it is only used when needed for contrast, emphasis, or clarity.

Also, Croatian often drops subject pronouns entirely because the verb ending already gives person and number:

• Polazi u sedam. = It leaves at seven. / Leaves at seven.
  This can work if the context already makes clear that you are talking about the train.

So using vlak here is the most natural full sentence.

Yes, vlak can feel tricky because of the consonant cluster vl-.

A rough guide:

• vlak sounds approximately like vlahk

Tips:

• Start with a quick v
• Move immediately into l
• Then a
• End with a crisp k

Do not insert an extra vowel at the beginning. Native English speakers may want to say something like vuh-lak, but Croatian says it as one compact syllable: vlak.