Sutra ću joj čestitati rođendan porukom i mahnuti joj kad stigne.

In Croatian, ću is a clitic, and clitics normally go in the second position of the clause.

So in Sutra ću joj čestitati..., the first element is Sutra, and the clitic ću comes right after it.

Compare:

• Ja ću joj čestitati... = I will congratulate her...
• Sutra ću joj čestitati... = Tomorrow I will congratulate her...

Both are normal. The difference is mostly about what comes first for emphasis or topic.

Yes, you can also say Čestitat ću joj rođendan...

Croatian future tense can appear in two common patterns:

• ću + infinitive → Sutra ću čestitati
• short infinitive + ću → Čestitat ću

When the clitic ću is forced into second position after another first word like Sutra, the full infinitive čestitati is used:

• Sutra ću čestitati

When the infinitive comes directly before ću, it often loses the final -i in writing:

• Čestitat ću

So the sentence is normal exactly as written.

Joj means to her.

It is the dative singular form of the pronoun ona (she). In this sentence, it marks the person receiving the action:

• čestitati joj rođendan = to congratulate her on her birthday / wish her a happy birthday
• mahnuti joj = to wave to her

So joj is the indirect object.

Because both verbs need it:

• čestitati joj
• mahnuti joj

In English, we often avoid repeating her if the meaning is obvious, but in Croatian repeating the pronoun is very natural and often clearer, especially when the sentence is a bit longer.

You might sometimes hear the second joj omitted in casual speech if the meaning is obvious, but repeating it here sounds perfectly normal.

Because čestitati has its own pattern of objects in Croatian:

• čestitati komu što

That means:

• komu = to whom → dative
• što = what → accusative

So:

• joj = to her (dative)
• rođendan = birthday (accusative object)

This is simply how the verb works in Croatian. English uses congratulate someone on something, but Croatian does not need a preposition here.

Here rođendan is in the accusative singular.

It looks the same as the nominative because rođendan is a masculine inanimate noun, and for many masculine inanimate nouns, nominative and accusative singular have the same form.

So:

• nominative: rođendan
• accusative: rođendan

The role is understood from the sentence structure, not from a different ending.

Porukom is the instrumental singular of poruka.

Here the instrumental expresses the means or method:

• porukom = by message / with a message

So it tells you how the speaker will congratulate her. In modern everyday use, this often suggests a text or written message, depending on context.

This is a question of aspect.

• mahnuti = perfective → one complete wave
• mahati = imperfective → waving repeatedly or continuously

In this sentence, the speaker means a single completed action in the future, so mahnuti fits better:

• mahnuti joj = wave to her once / give her a wave

If you wanted to describe ongoing waving, mahati would make more sense.

In Croatian, after words like kad (when), a verb in the present form is often used to refer to a future event, especially with a perfective verb.

So:

• kad stigne = when she arrives

This is the normal Croatian pattern. Using kad će stići here would usually sound unnatural in standard usage.

So although stigne looks like present tense, in this kind of clause it refers to a future action.

The subject is understood from context: it most naturally means when she arrives.

Croatian often leaves out subject pronouns when they are already clear. Since the sentence has already been talking about her with joj, the listener naturally understands that stigne refers to that same person.

If you needed to make it explicit, you could add the pronoun or noun, but usually it is not necessary.

Yes, very normal.

Croatian often drops subject pronouns because the verb form already gives enough information. Here ću tells you the speaker is talking about I will.

So Ja is not needed unless you want emphasis:

• Sutra ću joj čestitati... = neutral
• Ja ću joj čestitati... = more emphasis on I

Yes, Croatian word order is fairly flexible, but not completely free.

For example, you could also hear:

• Sutra ću joj porukom čestitati rođendan i mahnuti joj kad stigne.
• Joj ću sutra čestitati rođendan porukom... — possible in some contexts, but more marked

What stays important is:

• clitics like ću and joj tend to cluster near the beginning
• changes in word order usually change focus or emphasis, not the basic meaning

So the original sentence is natural, but not the only possible order.