Zabilježit ću datum sastanka u rokovnik čim primim poruku.

This is a normal Croatian future-tense pattern.

The full infinitive is zabilježiti. The future auxiliary is ću. When an infinitive ending in -ti comes directly before ću, it usually drops its final -i:

• zabilježiti + ću → zabilježit ću

So Zabilježit ću means I will note down / I’ll write down.

You can also put the auxiliary before the infinitive:

• Ja ću zabilježiti datum sastanka.

Both are correct. The version in your sentence is very common.

After time words like čim (as soon as), Croatian usually uses the present tense in the subordinate clause even when the meaning is future.

So:

• čim primim poruku = as soon as I receive the message

Croatian normally does not say:

• čim ću primiti poruku ✘

This is one of the big differences from English.

Because primim comes from the perfective verb primiti, which refers to a single completed event: to receive.

• primiti → primim
• primati → primam

Here the speaker means one completed future action: receiving one message. That is why primim is the natural choice.

Compare:

• čim primim poruku = as soon as I receive the message
• primam poruke svaki dan = I receive messages every day / I am receiving messages

So primam would sound ongoing, repeated, or habitual, which does not fit this sentence.

Sastanka is genitive singular of sastanak (meeting).

Croatian often expresses noun + noun relationships with the second noun in the genitive:

• datum sastanka = date of the meeting
• literally: date meeting-of

This is very common in Croatian:

• vrijeme sastanka = time of the meeting
• adresa hotela = address of the hotel
• broj telefona = phone number

So English often uses two nouns together, while Croatian often uses noun + genitive.

Because u can take different cases depending on the meaning.

• u + accusative often means into something, or putting something there
• u + locative often means in something, as a location

Here the idea is writing/entering the date into the planner, so Croatian uses:

• u rokovnik = into the planner

Compare:

• Zabilježit ću datum u rokovnik. = I’ll write the date into the planner.
• Datum je u rokovniku. = The date is in the planner.

So the difference is similar to into vs in.

Because poruku is the accusative singular form of poruka.

It is the direct object of primim:

• primiti što? → poruku

For many feminine nouns ending in -a, the accusative singular ends in -u:

• poruka → poruku
• knjiga → knjigu
• bilješka → bilješku

So this is a very regular pattern.

Not exactly.

• čim = as soon as, the moment
• kad / kada = when

Čim emphasizes immediacy: one action follows right after the other.

So:

• čim primim poruku = as soon as I receive the message

This feels more immediate than:

• kad primim poruku = when I receive the message

Both can be correct, but čim is better if you want the sense of immediately after.

Yes. Croatian word order is fairly flexible, although clitics like ću have placement rules.

Your sentence is natural:

• Zabilježit ću datum sastanka u rokovnik čim primim poruku.

But you could also say:

• Čim primim poruku, zabilježit ću datum sastanka u rokovnik.
• Datum sastanka zabilježit ću u rokovnik čim primim poruku.

The meaning stays basically the same, but the emphasis changes.

One important point: ću is a clitic, so it normally appears near the beginning of its clause, in the so-called second position.

Because Croatian has no articles like English a/an/the.

Croatian usually leaves definiteness to context.

So:

• datum sastanka can mean the date of the meeting
• poruku can mean the message or a message, depending on context

If the speaker wants to be more specific, Croatian uses other tools such as word order, context, demonstratives, or possessives:

• tu poruku = that message
• ovu poruku = this message

But very often nothing extra is needed.

Because Croatian often leaves out possessives when they are obvious from context.

So u rokovnik usually naturally suggests in my planner or in the planner, depending on the situation.

If you want to make it explicit, you can say:

• u svoj rokovnik = into my own planner
• u moj rokovnik = into my planner

But in ordinary speech, that is often unnecessary.

Zabilježiti means to note down, to make a note of, to record. It is a perfective verb, so it refers to one completed act.

It is close in meaning to zapisati, and in many contexts they are interchangeable.

Very roughly:

• zabilježiti can sound a bit like note down / record
• zapisati can sound a bit like write down

Also compare:

• bilježiti = to be noting down, to note repeatedly, to record over time
• zabilježiti = to note down once, successfully

So in this sentence, zabilježiti fits well because the speaker means one completed action.

Rokovnik is usually a planner, datebook, appointment book, or personal organizer.

It is not just any notebook. It usually suggests something used for dates, appointments, deadlines, or plans.

Grammatically, it is a masculine noun:

• rokovnik = nominative
• rokovnika = genitive
• rokovnik = accusative
• rokovniku = locative/dative

So u rokovnik is a very natural phrase for into the planner.

A few sounds may stand out to an English speaker:

• č in čim is like a harder ch sound, roughly like ch in chalk
• ć in ću is softer than č
• ž in zabilježiti sounds like the s in measure
• lj in zabilježiti is a single Croatian sound, not just plain l + j

You do not need perfect pronunciation immediately, but it helps to notice that Croatian distinguishes sounds that English often does not.