Povratna karta je potrebna.

• povratna = return, round-trip
• karta = ticket
• je = is
• potrebna = necessary, needed, required

So the structure is literally Return ticket is necessary.

Because povratna is an adjective, and it needs a noun to describe. Here it describes karta.

• povratna karta = return ticket
• jednosmerna karta = one-way ticket

In Serbian, this works much like English adjective + noun combinations, except the adjective changes its form to match the noun.

Because it agrees with karta, which is a feminine singular noun.

In Serbian, adjectives must match the noun in:

• gender
• number
• case

Since karta is feminine singular, both adjectives are feminine singular too:

• povratna karta
• karta je potrebna

Compare:

• pasoš je potreban = the passport is needed / a passport is needed
  (pasoš is masculine)
• viza je potrebna = a visa is needed
  (viza is feminine)
• odobrenje je potrebno = approval is needed
  (odobrenje is neuter)

It is in the nominative singular.

That is because povratna karta is the subject of the sentence:

• Povratna karta = what is needed
• je potrebna = is necessary

So this is a basic subject + verb + adjective sentence.

Je is the 3rd person singular form of biti = to be.

So:

• je = is

In standard Serbian prose, je is normally included:

• Povratna karta je potrebna.

In some styles, especially headlines, notes, or very compressed speech, Serbian may omit forms of to be, but for learners the normal sentence is with je.

Serbian has no articles like English a/an/the.

So karta can mean:

• a ticket
• the ticket

The exact meaning depends on context.

That means Povratna karta je potrebna could be understood as:

• A return ticket is required
• The return ticket is required

In many practical situations, English would translate this most naturally as A return ticket is required.

All three can work, depending on context.

• necessary = a more direct, dictionary-style meaning
• needed = common in everyday English
• required = often best in formal or travel-related situations

So in context, Povratna karta je potrebna is often best rendered as:

• A return ticket is required

But grammatically, potrebna itself basically means necessary/needed.

Yes. Serbian word order is more flexible than English.

The neutral order here is:

• Povratna karta je potrebna.

But you may also hear:

• Potrebna je povratna karta.

This version often sounds a little more like:

• A return ticket is needed or
• What is needed is a return ticket

Both are correct. The difference is mostly about emphasis and style, not basic meaning.

You negate je with nije:

• Povratna karta nije potrebna. = A return ticket is not required.

So:

• je = is
• nije = is not

This is a very useful pattern:

• Viza je potrebna. = A visa is required.
• Viza nije potrebna. = A visa is not required.

A rough English-friendly guide would be:

• Povratna ≈ poh-VRAHT-nah
• karta ≈ KAR-tah
• je ≈ yeh
• potrebna ≈ poh-TREB-nah

A few notes:

• r in Serbian is rolled or tapped.
• j is pronounced like English y in yes.
• Stress is not usually marked in normal writing, so learners often have to pick it up through listening.

Yes, povratna karta is the standard and very common expression.

Depending on region and context, you may also hear:

• povratna avionska karta = return plane ticket
• povratna autobuska karta = return bus ticket
• povratna karta za voz = return train ticket

But by itself, povratna karta is perfectly normal and widely understood.