Konac za zube je potreban.

Literally, konac za zube means thread for teeth.

Yes, it is a normal and understandable Serbian expression for dental floss. You may also hear zubni konac, which means tooth floss / dental floss and is a bit more compact. Both are fine.

• konac = thread, floss
• za = for
• zube = teeth

So the phrase is very transparent once you break it down.

Because Serbian does not have articles like English a/an and the.

So Konac za zube je potreban can mean things like:

• Dental floss is needed
• The dental floss is needed
• A dental floss thread is needed

In real use, the exact meaning depends on context.

This is very common in Serbian: nouns usually appear without any article at all.

Here za means for, and with this meaning it takes the accusative case.

The noun zub means tooth. Its plural forms include:

• nominative plural: zubi
• accusative plural: zube

So:

• za zube = for teeth

That is why you do not see zubi here.

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

So the sentence is built like this:

• Konac za zube = dental floss
• je = is
• potreban = needed / necessary

So structurally it is:

Dental floss + is + needed

In English, the verb is is required, and the same is true here in Serbian.

This is because je is a clitic, and Serbian clitics usually go in the second position of the clause.

Importantly, that means the second slot, not necessarily the second individual word.

In this sentence, konac za zube acts as one whole unit at the beginning, so je comes after that unit:

• Konac za zube | je | potreban

This may feel strange to an English speaker at first, but it is very normal in Serbian.

Because potreban has to agree with konac.

In Serbian, adjectives agree with the noun in gender, number, and usually case.
Here:

• konac is masculine
• konac is singular

So the adjective must also be masculine singular:

• potreban

Compare:

• Voda je potrebna. = Water is needed.
  (voda is feminine)
• Mleko je potrebno. = Milk is needed.
  (mleko is neuter)

So in your sentence, masculine konac requires potreban.

Yes. Potreban is an adjective meaning needed, necessary, or required.

In this sentence it is used predicatively, meaning it comes after je and describes the subject:

• Konac za zube je potreban.

This is similar to English:

• The floss is necessary
• The floss is needed

So even though it may feel a bit verbal in English, in Serbian it is grammatically an adjective here.

Yes, Serbian word order is more flexible than English word order.

For example, you could also say:

• Potreban je konac za zube.

This is also natural and may sound a bit more like What is needed is dental floss or Dental floss is needed with slight emphasis on potreban.

The original sentence:

• Konac za zube je potreban.

is perfectly correct too. The difference is mostly one of emphasis and style, not basic meaning.

It is correct, but in everyday conversation Serbian often uses trebati for personal need.

For example:

• Treba mi konac za zube. = I need dental floss.
• Potreban mi je konac za zube. = I need dental floss. / Dental floss is needed for me.

Your sentence, Konac za zube je potreban, sounds a bit more neutral, general, or formal—more like stating that dental floss is necessary.

So:

• general statement: Konac za zube je potreban
• personal need: Treba mi konac za zube

A rough English-friendly pronunciation is:

KO-nats za ZOO-be ye po-TRE-ban

A few notes:

• c in konac sounds like ts
• zube has a clear zoo sound at the start
• je is roughly like ye
• stress is not marked in normal writing, but a learner can say it naturally enough as
  KO-nats za ZOO-be ye po-TRE-ban

If you pronounce each vowel clearly, you will already sound much better:

• ko-nac
• za
• zu-be
• je
• po-tre-ban