Polazak je u osam, a povratak u sedam.

They are both nouns.

• polazak = departure, setting off
• povratak = return, coming back

So the sentence uses nouns, not verbs like we leave / we return. Serbian often uses this kind of noun-based phrasing in schedules, announcements, and travel contexts.

Because the second je is simply omitted as understood.

The full version would be:

Polazak je u osam, a povratak je u sedam.

In Serbian, when two clauses are parallel, the repeated verb is often left out in the second one to avoid sounding repetitive. English does this too sometimes:

• Departure is at eight, and return at seven.

So the omission is natural and normal here.

A often links two ideas by contrasting or balancing them.

Here it works like:

• while
• and as for
• whereas

So Polazak je u osam, a povratak u sedam feels like:

• departure is at eight, while return is at seven

It is not a strong contradiction, so ali would be too strong.
And i would just mean and, without that balancing/contrastive feel.

In Serbian, specific clock times are normally expressed with u:

• u jedan
• u tri
• u sedam
• u osam

So u osam means at eight o’clock, and u sedam means at seven o’clock.

This is just the standard pattern for saying when something happens at a specific hour.

By itself, u osam just means at eight. It does not automatically tell you morning or evening.

You understand it from context, or you can make it explicit:

• u osam ujutru = at 8 in the morning
• u osam uveče = at 8 in the evening

So the sentence leaves that unspecified unless the situation already makes it clear.

Yes. That is also correct.

• u osam = at eight
• u osam sati = at eight o’clock

The word sati is often optional when the meaning is already clear. In everyday speech, people very often just say u osam, u sedam, etc.

Yes, absolutely.

Both are correct:

• Polazak je u osam, a povratak u sedam.
• Polazak je u osam, a povratak je u sedam.

The version without the second je is a little more concise and natural in this kind of paired statement.

Yes. Serbian word order is flexible.

These are all possible:

• Polazak je u osam.
• U osam je polazak.

The basic meaning stays the same, but the emphasis changes a little.
Starting with u osam puts more focus on the time.
Starting with polazak puts more focus on the event itself.

In schedules and neutral statements, Polazak je u osam is very natural.

Yes. They are in the nominative because they are the main nouns being talked about:

• Polazak je...
• Povratak (je)...

They function like the subject/topic of each clause. That is why you see the dictionary form of the nouns here.