Lekarsko uverenje je potrebno za produženje.

• lekarsko = medical, doctor’s
• uverenje = certificate, attestation
• je = is
• potrebno = necessary, needed, required
• za = for
• produženje = extension, renewal

A very literal breakdown is Doctor’s/medical certificate is necessary for extension/renewal.

Because uverenje is a neuter singular noun, and Serbian words that describe it have to agree with it.

So:

• lekarsko uverenje = medical certificate
• uverenje je potrebno = the certificate is necessary

Both lekarsko and potrebno are in the neuter singular form to match uverenje.

Compare:

• lekarska potvrda je potrebna = a medical confirmation is necessary
  feminine
• lekarski pregled je potreban = a medical exam is necessary
  masculine

Je is the 3rd person singular of biti = to be, and in Serbian it often behaves like a clitic, which means it tends to go in the second position of the sentence or clause.

So in:

• Lekarsko uverenje je potrebno za produženje

the first unit is Lekarsko uverenje, and je comes right after it.

This is very normal in Serbian word order.

Serbian has no articles. There is no direct equivalent of English a/an/the.

So lekarsko uverenje can mean:

• a medical certificate
• the medical certificate

The exact meaning depends on context.

Here za means for, and it takes the accusative.

So za produženje is for renewal/for extension.

The useful thing to notice is that produženje is a neuter noun, and its nominative and accusative singular look the same, so you do not see a change in the form.

It can mean either, depending on context.

In bureaucratic language, produženje is often used for:

• extending a permit
• renewing a document
• prolonging a deadline or period

So in English, the best translation may be extension or renewal, depending on what is being talked about.

Yes, it is a very common official or administrative phrase. It usually means a formal medical certificate from a doctor.

You may also hear lekarska potvrda, which is similar, but uverenje often sounds a bit more formal or bureaucratic.

So in official contexts, lekarsko uverenje is a very natural phrase.

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

For example, you can also say:

• Za produženje je potrebno lekarsko uverenje.
• Potrebno je lekarsko uverenje za produženje.

These all mean roughly the same thing, but the emphasis changes a little:

• starting with za produženje emphasizes the purpose
• starting with potrebno emphasizes the requirement

Лекарско уверење је потребно за продужење.

Serbian normally uses both Latin and Cyrillic, and this is the exact same sentence in the other script.

A rough pronunciation guide is:

LE-kar-sko u-VE-ren-ye ye po-TREB-no za pro-DU-zhe-nye

A few helpful sound notes:

• j sounds like English y in yes
• ž sounds like the s in measure
• nj sounds like ny in canyon

So:

• uverenje sounds roughly like u-VE-ren-ye
• produženje sounds roughly like pro-DU-zhe-nye