Bilo bi pristojno da se javiš ako kasniš.

Bilo bi is the conditional of biti (to be), literally it would be.

So Bilo bi pristojno... means It would be polite...

Croatian often uses this kind of conditional to sound:

• less direct,
• more polite,
• more like advice than a command.

Compare:

• Pristojno je... = It is polite...
• Bilo bi pristojno... = It would be polite...

In this sentence, it softens the statement and makes it sound more tactful.

Pristojno is the neuter singular form of the adjective pristojan (polite, decent).

It is neuter because Croatian is using an impersonal construction, similar to English It would be polite...

There is no real concrete noun here. The adjective is not describing a masculine or feminine person; it is describing the whole idea/action. So Croatian uses the neuter form:

• pristojan = masculine
• pristojna = feminine
• pristojno = neuter

This is very common in Croatian with general statements:

• Dobro je učiti. = It is good to study.
• Važno je doći na vrijeme. = It is important to arrive on time.
• Bilo bi pristojno... = It would be polite...

Here da introduces a subordinate clause: da se javiš.

In English, we often use an infinitive after expressions like it would be polite:

• It would be polite to call / to let someone know

Croatian very often uses da + present tense instead of an infinitive in this kind of sentence:

• Bilo bi pristojno da se javiš...

So a good way to think of it is:

• da = introducing that / to-type content, depending on context

You do not translate it word-for-word every time. Its job here is grammatical: it connects it would be polite with you let someone know.

Javiti se is a very common verb meaning things like:

• to get in touch
• to let someone know
• to report in
• to call/message/check in

In this sentence, it most naturally means:

• to let someone know
• to contact the person

So da se javiš ako kasniš means something like:

• that you let someone know if you are late
• that you get in touch if you’re running late

The exact English wording depends on context, but the Croatian verb is very natural here.

Yes, the dictionary form is javiti se.

Sometimes se is truly reflexive, but often in Croatian it is just part of the verb and you need to learn it together with the verb. Here, javiti se is best learned as one unit.

Very roughly:

• javiti can mean announce / inform
• javiti se often means get in touch / make contact / say hello / report oneself

Examples:

• Javio sam mu vijest. = I informed him of the news.
• Javio sam se. = I called / got in touch / checked in.

So in this sentence, do not try to interpret se too literally as yourself. Just memorize javiti se as the normal expression.

This is because se is a clitic, and Croatian clitics usually go in a special early position in the clause.

After conjunctions like da, clitics often come immediately after it:

• da se javiš
• da mi kažeš
• da ga vidiš

So da se javiš is the normal word order.

Da javiš se sounds wrong in standard Croatian.

Because Croatian commonly uses the present tense in subordinate clauses after da and ako, even when English might use other structures.

Here:

• javiš = you let someone know / you get in touch
• kasniš = you are late / you’re running late

They are both:

• 2nd person singular
• present tense

This is completely normal Croatian grammar:

• Bilo bi dobro da dođeš. = It would be good if you came / for you to come.
• Reci mi ako trebaš pomoć. = Tell me if you need help.

So even though English might use to call or if you’re late, Croatian simply uses present-tense verb forms.

Javiš is:

• 2nd person singular
• present tense
• from javiti se

So it means you get in touch / you let someone know.

A quick mini-paradigm:

• ja se javim = I get in touch
• ti se javiš = you get in touch
• on/ona se javi = he/she gets in touch

In this sentence, ti is not stated because Croatian often omits subject pronouns when the verb ending already shows the person.

Kasniš is:

• 2nd person singular
• present tense
• from kasniti = to be late

So:

• ja kasnim = I am late
• ti kasniš = you are late
• on/ona kasni = he/she is late

Again, the pronoun ti is omitted because it is already clear from the ending -iš.

It can cover both, depending on context.

Ako kasniš literally looks like if you are late, but in natural use it can also mean:

• if you’re running late
• if you’re going to be late

Croatian present tense often has a slightly broader use than a strict word-for-word English present tense.

So in real life, this sentence could be used in a situation where someone has not yet arrived and may be delayed.

Yes. These forms are 2nd person singular informal, used with ti.

So this sentence is addressed to one person informally:

• Bilo bi pristojno da se javiš ako kasniš.

If you wanted to say the same thing formally, or to more than one person, you would use vi forms:

• Bilo bi pristojno da se javite ako kasnite.

So:

• javiš / kasniš = informal singular
• javite / kasnite = formal singular or plural

Yes, absolutely.

Both are natural:

• Bilo bi pristojno da se javiš ako kasniš.
• Ako kasniš, bilo bi pristojno da se javiš.

The meaning stays basically the same. The difference is mostly emphasis:

• Bilo bi pristojno... first puts focus on the judgment/advice.
• Ako kasniš... first puts focus on the condition.

Croatian word order is more flexible than English, but not completely free. These two versions are both normal.

It usually sounds like a polite reproach, advice, or a mild criticism, depending on tone.

It is softer than a direct command such as:

• Javi se ako kasniš. = Let me know if you’re late.

But it can still imply:

• You really should do this
• That would be the proper thing to do

So the sentence is grammatically polite, but in context it may still sound a little pointed if someone has already failed to do it.

Not naturally in this sentence.

English says:

• It would be polite to let someone know...

But Croatian strongly prefers:

• Bilo bi pristojno da se javiš...

Using an infinitive here would sound unnatural or unidiomatic. This is one of those places where Croatian structure differs from English, so it is best to learn the pattern as a whole:

• bilo bi + adjective + da + present tense

Examples:

• Bilo bi dobro da dođeš. = It would be good for you to come.
• Bilo bi korisno da pročitaš to. = It would be useful for you to read that.
• Bilo bi pristojno da se javiš. = It would be polite to let someone know.

Yes, in a way.

English requires it:

• It would be polite...

Croatian does not need a dummy subject here. The sentence is simply:

• Bilo bi pristojno...

So you can think of it as an impersonal sentence. The idea of English it is understood, but Croatian does not have to say anything corresponding to it.

That is why the adjective stays in the neuter singular:

• pristojno

It matches the impersonal, general statement rather than any real noun.