Moji roditelji će stići u Zagreb u petak navečer.

Because moji has to agree with roditelji in gender, number, and case.

• roditelji = nominative plural
• it is treated as masculine plural
• so the correct form is moji

Compare:

• moj otac = my father
• moja majka = my mother
• moji roditelji = my parents

In Croatian, a mixed group normally takes masculine plural agreement.

So even though parents includes one male and one female, the plural noun roditelji behaves grammatically as masculine plural.

Also:

• roditelj = parent
• roditelji = parents

Croatian commonly forms the future with:

clitic form of htjeti + infinitive

So here:

• će = future auxiliary clitic
• stići = infinitive, to arrive / to reach

Together, će stići means will arrive.

A useful thing to know: će is used for both he/she/it will and they will. The subject tells you which one it is.

• On će stići = He will arrive
• Oni će stići = They will arrive

Because će is a clitic. In Croatian, clitics usually cannot stand alone at the beginning of a clause and tend to appear in second position.

So Moji roditelji će stići is a normal, neutral word order.

This is one of the things English speakers often need time to get used to, because Croatian clitic placement follows rules that English does not have.

Because after the future auxiliary će, Croatian normally uses the infinitive.

So:

• će stići = will arrive
• not a finite present-tense form after će

This is different from English, where you say will arrive without thinking of arrive as an infinitive. In Croatian grammar, though, this is analyzed as the infinitive.

stići means to arrive, to get there, or to reach a destination. It often suggests successfully making it to the place.

It is close in meaning to doći, and in many contexts both can be translated as to come / to arrive. But stići often puts a bit more focus on reaching the destination.

It is also a perfective verb, which makes sense here because this is a single completed future event.

Because Croatian uses different cases depending on whether you mean movement to a place or location in a place.

• u + accusative = movement into / to
• u + locative = location in

So:

• u Zagreb = to Zagreb
• u Zagrebu = in Zagreb

Here the parents are going to Zagreb, so Croatian uses u Zagreb.

Also, Zagreb is a masculine inanimate noun, and its accusative looks the same as its nominative, so you do not see a changed ending.

Because Croatian also uses u in some time expressions.

Here:

• u petak = on Friday

So the sentence has:

• u Zagreb = to Zagreb
• u petak = on Friday

Same preposition, different function.

In u petak, the noun petak is in the accusative singular.

Both are possible, but they mean different things.

• u petak = on Friday, one specific Friday
• petkom = on Fridays, habitually / every Friday

So in your sentence, u petak is correct because it refers to one particular Friday.

navečer is an adverb meaning in the evening or toward evening.

So:

• u petak navečer = on Friday evening

Because navečer is already an adverb, it does not need an extra preposition here.

English speakers often expect something more word-for-word like in Friday evening, but Croatian packages this idea differently.

They are related, but not the same.

• navečer = in the evening
• večeras = this evening / tonight

So in this sentence, navečer fits because the evening is being identified by u petak.

If you used večeras, it would mean tonight, relative to the moment of speaking, which is a different idea.

Yes. Croatian word order is fairly flexible, and speakers often move parts of the sentence for emphasis, topic, or style.

The version you have is a very natural, neutral one:

Moji roditelji će stići u Zagreb u petak navečer.

Other orders are possible, but they may sound more marked or shift the focus. Even when word order changes, clitics like će still have to be placed naturally.

Because Croatian does not have articles like English a and the.

So Croatian simply says:

• moji roditelji = my parents
• u Zagreb = to Zagreb
• u petak navečer = on Friday evening

Whether something is definite or indefinite usually comes from context, not from an article.