Ako budeš imao pitanje, javi mi se porukom.

Budeš imao is Future II in Croatian.

It is formed with:

• budeš = 2nd person singular of biti
• imao = past active participle of imati

In sentences like this, Future II is very commonly used after words such as:

• ako = if
• kad = when
• čim = as soon as

So Ako budeš imao pitanje means something like:

• if you have a question
• if you should happen to have a question
• if you end up having a question

It sounds natural for a future possible situation.

Both are possible, but they are not exactly the same in nuance.

• Ako budeš imao pitanje sounds more clearly connected to the future.
• Ako imaš pitanje sounds more general, or more immediate, like if you have a question in a general sense.

So:

• Ako budeš imao pitanje, javi mi se porukom.
  More future-oriented, very natural in instructions or offers.

• Ako imaš pitanje, javi mi se porukom.
  Also correct, a little simpler and slightly less specifically future.

A learner should understand both.

Yes. Imao agrees with the gender of the person being addressed.

• to a man: Ako budeš imao pitanje
• to a woman: Ako budeš imala pitanje

For plural or formal you:

• Ako budete imali pitanje

So the participle changes:

• imao = masculine singular
• imala = feminine singular
• imali = plural or formal

Javi mi se comes from the verb javiti se.

A natural English equivalent is:

• contact me
• get in touch with me
• message me
• reach out to me

The parts are:

• javi = imperative form of javiti
• se = part of the reflexive verb javiti se
• mi = to me

So literally it is something like:

• get in touch with me

This is a very common and natural phrase in Croatian.

They do different jobs.

• se belongs to the verb javiti se
• mi means to me

So:

• javi mi se = contact me / get in touch with me

If you remove se, the meaning changes.

Compare:

• Javi mi se. = Get in touch with me.
• Javi mi nešto. = Tell me something / let me know something.
• Javi mi odgovor. = Tell me the answer.

So se is not just extra decoration here. It is part of the meaning.

Because Croatian has a fairly fixed order for unstressed short words called clitics.

Here:

• mi is a clitic
• se is also a clitic

In standard word order, mi comes before se.

So:

• Javi mi se = correct
• Javi se mi = not standard

This is one of those patterns that learners usually need to get used to rather than translate word by word.

Because porukom is instrumental singular of poruka, and here it shows the means or method.

So porukom means:

• by message
• via message

That is why the sentence means:

• contact me by message

Compare:

• javi mi se porukom = get in touch with me by message
• pošalji mi poruku = send me a message

In the second sentence, poruku is the direct object, so it is in the accusative.

Usually it means some kind of written message, depending on context:

• text message
• chat message
• direct message
• app message

In modern everyday Croatian, poruka is very flexible. It can refer to:

• SMS
• WhatsApp message
• Viber message
• Messenger message
• similar digital messages

If needed, Croatian can be more specific:

• SMS-om
• u poruci
• na WhatsAppu
• tekstualnom porukom

But porukom by itself is very natural.

Because Ako budeš imao pitanje is a subordinate clause introduced by ako.

In Croatian, clauses like this are separated by a comma:

• Ako budeš imao pitanje, javi mi se porukom.

This is standard punctuation.

English is similar here:

• If you have a question, message me.

So the comma is not optional in normal standard writing.

This is about aspect.

• javiti se is perfective
• javljati se is imperfective

The imperative javi se suggests:

• one completed action
• contact me once, when needed

That fits this sentence well.

By contrast:

• javljaj mi se would suggest repeated or ongoing contact
• keep getting in touch with me
• check in with me from time to time

So here javi mi se is the natural choice.

Use the plural/formal you forms:

• Ako budete imali pitanje, javite mi se porukom.

That is the polite version when addressing one person formally or several people.

You may also often hear the plural noun:

• Ako budete imali pitanja, javite mi se porukom.

That means:

• If you have any questions, message me.

In many real-life situations, pitanja sounds even more natural than singular pitanje, especially in customer service, teaching, or business communication.