Moram prijaviti promjenu adrese nakon selidbe.

In Croatian, the subject pronoun is often omitted because the verb ending already shows who the subject is.

• moram = I must
• the ending -am tells you it is 1st person singular

So Moram prijaviti... is the natural way to say I must report/register...
You can add ja for emphasis, but it is not normally necessary.

Because after morati (to have to / must), Croatian normally uses an infinitive.

So the pattern is:

• moram + infinitive
• moram prijaviti = I must report/register

This is similar to English must report, except Croatian does not need to here.

Examples:

• Moram ići. = I must go.
• Moram platiti. = I must pay.

Here prijaviti means something like to report, to register, or to officially declare.

In this sentence, it refers to officially notifying the authorities or some institution about a change of address.

A useful idea:

• javiti = to let someone know / inform
• prijaviti = to officially report or register something

So prijaviti promjenu adrese is more formal and administrative than simply reći or javiti.

Because promjenu is the accusative singular form of promjena (change).

It is in the accusative because it is the direct object of prijaviti:

• prijaviti što? → promjenu

Declension:

• nominative: promjena
• accusative: promjenu

This is very common with feminine nouns ending in -a:

• knjiga → knjigu
• adresa → adresu
• promjena → promjenu

Because adrese is in the genitive singular after the noun promjena.

Croatian often uses this pattern:

• promjena + genitive = change of ...

So:

• promjena adrese = change of address
• in the sentence, since promjena becomes accusative (promjenu), adrese stays genitive:
  • prijaviti promjenu adrese

Compare:

• promjena plana = change of plan
• promjena imena = change of name

Because the preposition nakon (after) requires the genitive case.

The noun is:

• nominative: selidba = move / moving / relocation

After nakon, it becomes:

• genitive: selidbe

So:

• nakon selidbe = after the move / after moving

This is a fixed grammar rule:

• nakon + genitive

Yes. Poslije selidbe is also correct and means the same thing: after the move.

Both are common:

• nakon selidbe
• poslije selidbe

Very roughly:

• nakon can sound a little more neutral or formal
• poslije is very common in everyday speech

In this sentence, either one works well.

No, Croatian word order is fairly flexible, although some versions sound more natural than others.

The original:

• Moram prijaviti promjenu adrese nakon selidbe.

You could also hear:

• Nakon selidbe moram prijaviti promjenu adrese.

That version puts more focus on after the move.

Because Croatian uses case endings, word order often shows emphasis rather than basic grammar. Still, the original sentence is very natural and neutral.

It is a noun. Selidba means a move, relocation, or moving as an event.

So:

• nakon selidbe literally means after the relocation / after the move

Croatian often uses nouns where English might use either a noun or an -ing form.

Related words:

• seliti se = to move house / relocate
• selidba = move, moving, relocation

Yes, it matters. Prijaviti is perfective.

That means it refers to completing the action: making the report/registration successfully.

Its imperfective partner is usually:

• prijavljivati

So:

• prijaviti = to report/register once, to completion
• prijavljivati = to be reporting, to report repeatedly/habitually

With moram, the perfective form is very natural here because the sentence is about a single necessary completed action:

• Moram prijaviti promjenu adrese. = I have to file/report the address change.

Yes, in this context it almost certainly means a physical/postal address.

Because the sentence includes:

• promjenu adrese = change of address
• nakon selidbe = after moving

That strongly points to a home or residence address.

Technically, adresa can also mean other kinds of address depending on context, but here the meaning is clearly a residential/postal one.