Potpisni certifikat služi za elektronički potpis.

Literally, potpisni certifikat is something like signature certificate or signing certificate.

In Croatian, it is very common to express this kind of idea with an adjective + noun structure:

• potpis = signature
• potpisni = signature-related, signing

So potpisni certifikat is not a full clause like certificate for signing, but a compact noun phrase where potpisni describes what kind of certificate it is.

This is very natural in Croatian. English often uses noun + noun combinations, but Croatian usually prefers an adjective where possible.

Certifikat is in the nominative singular.

That is because it is the subject of the sentence:

• Potpisni certifikat = the signing certificate
• služi = serves / is used

So the structure is:

• Potpisni certifikat = subject
• služi = verb
• za elektronički potpis = purpose

In other words, the certificate is the thing doing the serving/being used, so it stays in the nominative.

Služi is the 3rd person singular present tense of služiti.

In this sentence, it means:

• serves
• is used
• is intended

So:

• Potpisni certifikat služi za elektronički potpis. = A signing certificate is used for an electronic signature.

A very helpful pattern to remember is:

• služiti za + accusative = to serve for / to be used for

Here za expresses purpose: what something is for.

So:

• služiti za nešto = to be used for something
• služi za elektronički potpis = it is used for an electronic signature

This is a very common Croatian pattern.

Compare:

• Ovaj alat služi za rezanje. = This tool is used for cutting.
• Program služi za obradu podataka. = The program is used for data processing.

The preposition za here requires the accusative case.

So elektronički potpis is in the accusative singular.

However, for masculine inanimate nouns, the accusative singular is often the same as the nominative singular. That is exactly what happens here:

• nominative: elektronički potpis
• accusative: elektronički potpis

So even though the case has changed grammatically, the form looks the same.

Because it agrees with potpis in gender, number, and case.

• potpis is masculine singular
• here it is accusative singular, but since it is inanimate, it looks like the nominative
• the adjective must match it

So:

• elektronički potpis = masculine singular

This same agreement rule applies throughout Croatian:

• dobar program
• novi dokument
• elektronički potpis

The adjective changes depending on the noun it describes.

• potpis = signature (a noun)
• potpisni = signature-related / signing (an adjective)

So:

• potpis by itself names the thing
• potpisni describes another noun

Compare:

• potpis = signature
• potpisni certifikat = signing certificate
• potpisna kartica = signature card

This is similar to English pairs like:

• nation → national
• music → musical (not identical in formation, but similar in function)

Yes, Croatian word order is more flexible than English, but the neutral, most natural order here is:

• Potpisni certifikat služi za elektronički potpis.

You could change the order for emphasis, for example:

• Za elektronički potpis služi potpisni certifikat.

That is still grammatical, but it sounds more marked and emphasizes what it is used for.

For a learner, the original version is the best one to use as your default pattern.

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

So certifikat can mean:

• a certificate
• the certificate

The exact meaning depends on context.

That means:

• Potpisni certifikat služi za elektronički potpis.

could be understood in English as:

• A signing certificate is used for an electronic signature.
• The signing certificate is used for an electronic signature.

Both are possible, depending on the situation.

Yes. Elektronički potpis is the standard and natural Croatian term for electronic signature.

You may also see related terms such as:

• digitalni potpis = digital signature
• elektronički potpis = electronic signature

In many technical or legal contexts, the distinction may matter, but in general learning terms, elektronički potpis is the normal phrase used in the sentence here.

A very natural English translation would be:

• A signing certificate is used for an electronic signature.

Depending on context, you might also say:

• The signing certificate is used for electronic signing.
• A signature certificate is used for an electronic signature.

The Croatian sentence is compact and formal-sounding, which is common in technical language.