Poziv na koncert je u poruci.

Because poziv is the subject of the sentence, so it stays in the nominative case.

• poziv = invitation / call
• nominative singular: poziv
• genitive singular: poziva

In Poziv na koncert je u poruci, the sentence is basically:

• The invitation = subject
• is = verb
• in the message = location

So poziv has to be nominative.

Je is the 3rd person singular present tense of the verb biti (to be).

So:

• je = is

In Serbian, unlike in some present-tense sentences with adjectives or nouns in certain styles, here the verb je is normally used:

• Poziv na koncert je u poruci.
• The invitation to the concert is in the message.

It links the subject poziv na koncert with the location u poruci.

Because Serbian uses na with certain events and destinations to mean to or for in English.

Here, poziv na koncert means:

• an invitation to the concert

The preposition na here takes the accusative case.

So:

• koncert is in the accusative singular

However, for many masculine inanimate nouns, the accusative looks the same as the nominative:

• nominative: koncert
• accusative: koncert

So the form does not change, even though the case does.

Because poziv na + event/place/activity is the normal Serbian pattern for invitation to ...

Examples:

• poziv na koncert = invitation to the concert
• poziv na večeru = invitation to dinner
• poziv na sastanak = invitation to the meeting

Using za koncert would sound unnatural here. Za often means for, but Serbian does not use it in this expression.

Because the preposition u means in, and when it shows location, it takes the locative case.

The noun is:

• poruka = message

Its locative singular is:

• u poruci = in the message

So the change is:

• poruka → poruci

This is a common pattern for many feminine nouns ending in -a.

It is the locative singular of poruka.

Why locative?

Because u can take two different cases:

• u + accusative = movement into something
• u + locative = being inside something / location

Here the sentence means the invitation is located in the message, not moving into it. So Serbian uses:

• u poruci = in the message

Compare:

• Stavio sam poziv u poruku. = I put the invitation into the message.
  (u + accusative)
• Poziv je u poruci. = The invitation is in the message.
  (u + locative)

It usually means message in a broad sense. The exact type depends on context.

So u poruci could mean:

• in a text message
• in a chat message
• in an email/message
• in a written message in general

If the wider context is about phones, many learners will naturally understand it as in the text message.

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

So:

• poziv can mean an invitation or the invitation
• poruka can mean a message or the message

The exact meaning comes from context.

In this sentence, English may translate it as The invitation to the concert is in the message, but Serbian does not need a separate word for the.

No, Serbian word order is fairly flexible.

The sentence:

• Poziv na koncert je u poruci.

is a normal, neutral way to say it.

But you could also say:

• U poruci je poziv na koncert.

This puts more focus on u poruci (in the message).

Because Serbian uses cases, word order can shift more easily than in English. Still, not every order sounds equally natural, and the most neutral version here is the original one.

Yes, but the meaning changes slightly.

• poziv = invitation, summons, call
• pozivnica = invitation card / formal invitation

So:

• poziv na koncert = invitation to the concert
• pozivnica za/na koncert can mean an actual invitation card or ticket-like invitation, depending on context

In your sentence, poziv is a general and natural choice if you just mean that the message contains the invitation.

Yes, it works as a noun phrase:

• poziv = head noun
• na koncert = prepositional phrase describing what kind of invitation it is

Together:

• poziv na koncert = invitation to the concert

This whole phrase functions as the subject of the sentence.

Yes.

For example:

• Poziv na koncert u poruci je... would usually sound incomplete or mean something different, so be careful.
• But U poruci je poziv na koncert is perfectly natural.

In Serbian, short forms like je often appear in the second position of the clause, which is why in U poruci je poziv na koncert, the je comes right after the first phrase.

A very literal breakdown is:

• Poziv = invitation
• na koncert = to the concert
• je = is
• u poruci = in the message

So the structure is:

• [Invitation to the concert] [is] [in the message].

That is a very typical Serbian sentence pattern:
subject + verb + location phrase.