Korisnička podrška obavila je razgovor.

Because podrška is a feminine singular noun, and the adjective must agree with it.

• korisnički = basic dictionary form / masculine singular
• korisnička = feminine singular
• korisničko = neuter singular

So:

• korisnička podrška = customer support

This is normal adjective–noun agreement in Croatian.

It is a noun phrase made up of:

• korisnička = adjective, feminine singular nominative
• podrška = noun, feminine singular nominative

Together they form the subject of the sentence.

Literally, it is something like user support or customer support, depending on context.

Because it is in the nominative case, which is the usual case for the subject of the sentence.

In this sentence:

• Korisnička podrška = the subject
• obavila je = the verb
• razgovor = the object

So the structure is basically:

[Subject] + [Verb] + [Object]

Obavila je is the perfect tense.

It is made of two parts:

• obavila = past active participle
• je = present-tense form of biti (to be), 3rd person singular

So Croatian forms the perfect like this:

• obavila je = has done / did
• masculine: obavio je
• feminine: obavila je
• neuter: obavilo je

Since the subject podrška is feminine singular, the participle must also be feminine singular: obavila.

In Croatian, the auxiliary je is a clitic, and clitics usually go in the second position of the clause.

That is why Croatian often says:

• Podrška je obavila razgovor.
• Korisnička podrška obavila je razgovor.

Both are possible, but clitic placement follows special word-order rules, not the same rules as English.

In your sentence, je comes after obavila, which is a normal and correct placement.

Yes. That is also correct and probably the version many learners first expect.

Both are grammatical:

• Korisnička podrška je obavila razgovor.
• Korisnička podrška obavila je razgovor.

The difference is mainly one of style, rhythm, and information structure, not basic meaning.

Here razgovor is in the accusative singular because it is the direct object of the verb.

The tricky part is that for many inanimate masculine nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: razgovor
• accusative: razgovor

That is why its form does not change here.

Because razgovor is a masculine inanimate noun.

In Croatian:

• masculine animate nouns usually change in the accusative
• masculine inanimate nouns usually stay the same as the nominative

Compare:

• vidim čovjeka = I see the man
  (čovjek → čovjeka)
• vodim razgovor = I am conducting a conversation
  (razgovor stays razgovor)

So razgovor does not change because it is inanimate.

Obaviti usually means something like:

• to complete
• to carry out
• to conduct
• to get done

In this sentence, it suggests that the conversation was carried out/completed as a whole.

That is important because obaviti is a perfective verb. Perfective verbs present the action as completed.

Its imperfective partner is obavljati.

So roughly:

• obavljati = to be doing / to do regularly
• obaviti = to do, complete

Because obavljala je would be the imperfective past, focusing more on the process or repeated action.

Compare:

• obavila je razgovor = she/it completed the conversation
• obavljala je razgovor = she/it was conducting the conversation / used to conduct conversations

In your sentence, the speaker is presenting the action as a finished whole, so obavila je is the natural choice.

Yes, it is grammatical and understandable.

However, depending on context, a native speaker might also say something slightly different, for example:

• Korisnička podrška obavila je poziv. = Customer support made/completed the call.
• Korisnička podrška vodila je razgovor. = Customer support conducted the conversation.
• Korisnička podrška razgovarala je s korisnikom. = Customer support spoke with the customer.

So your sentence is fine, but the most natural wording can depend on whether the context is a phone call, a formal interview, or a general conversation.

Because Croatian has no articles.

English uses:

• a
• an
• the

Croatian does not. So a noun like razgovor can mean:

• a conversation
• the conversation

The exact meaning depends on context.

The same is true for korisnička podrška, which can mean customer support in a general or specific sense depending on the situation.