Kad budemo opet na obali, pokazat ću ti stijenu kraj koje je bila ona bijela jedrilica.

Because after conjunctions like kad (when), Croatian normally uses future II in the subordinate clause when talking about a future situation.

So:

• Kad budemo opet na obali... = When we are back at the coast / when we find ourselves at the coast again...
• Main clause: pokazat ću ti... = I will show you...

This is a very common pattern:

• Kad dođeš, nazovi me. = When you come, call me.
• Kad budemo spremni, krenut ćemo. = When we are ready, we’ll leave.

Using kad ćemo biti here would sound unnatural in standard Croatian.

Budemo is the 1st person plural form of the verb biti (to be) in future II.

It corresponds to we will be in this sentence.

The subject is not written because Croatian often leaves pronouns out when the verb already shows the person:

• budemo = we will be
• implied subject: mi

So Kad budemo opet na obali literally means When we will be again on the coast, but in natural English that becomes When we’re on the coast again.

Because na can take different cases depending on meaning:

• na + accusative = movement onto/to
• na + locative = location on/at

Here the meaning is location, not motion:

• na obali = on/at the coast
• not toward the coast

Compare:

• Idemo na obalu. = We’re going to the coast.
• Mi smo na obali. = We are at the coast.

So Kad budemo opet na obali means When we’re at the coast again.

Obali is locative singular of obala (coast, shore).

The basic form is:

• obala = nominative singular

After na in a location meaning, you use the locative:

• na obali = on/at the coast

Both are related to the Croatian future I.

The full infinitive is pokazati. With the clitic auxiliary ću, Croatian often places the auxiliary in second position, and then the infinitive loses its final -i:

• pokazati + ću → pokazat ću

So:

• Pokazat ću ti... = I will show you...

You can also say:

• Ja ću pokazati...

But in sentences like this, pokazat ću is very normal and natural.

Ti here is the dative singular form of ti (you, singular/informal), and it means to you.

So:

• pokazat ću ti stijenu = I will show you the rock

Literally:

• pokazat ću = I will show
• ti = to you
• stijenu = the rock

Croatian often uses the dative for the person something is shown, given, said, etc.

Because stijenu is the accusative singular of stijena (rock, cliff, depending on context), and it is the direct object of pokazati.

• nominative: stijena
• accusative: stijenu

So:

• pokazat ću ti stijenu = I will show you the rock

This is normal for feminine nouns ending in -a:

• kuća → kuću
• žena → ženu
• stijena → stijenu

Here kraj is a preposition meaning beside, next to, or near.

So:

• kraj koje je bila... = next to which there was...

Learners often first meet kraj as a noun meaning end, but in this sentence it is definitely the preposition.

As a preposition, kraj takes the genitive case.

Because kraj requires the genitive, and koje is the genitive singular feminine form of the relative pronoun.

The pronoun refers back to stijenu:

• stijena is feminine singular

But the case of the relative pronoun is determined by its role inside the relative clause, not by the case of stijenu in the main clause.

Here the structure is:

• kraj + genitive
• therefore: kraj koje

So:

• stijenu kraj koje je bila ona bijela jedrilica
• literally: the rock next to which that white sailboat was

If it were kraj koju, that would be wrong because kraj does not take the accusative here.

Because the relative pronoun must agree with the noun it refers to in gender and number, but its case depends on its function in the relative clause.

It refers to:

• stijenu ← from stijena
• gender: feminine
• number: singular

Inside the relative clause, it comes after kraj, which requires genitive. So you get:

• feminine singular + genitive = koje

That is why the form is koje, even though the noun in the main clause is stijenu.

Because the subject of that relative clause is ona bijela jedrilica (that white sailboat), which is:

• feminine
• singular

So the verb must also be feminine singular in the past tense:

• je bila = was

The relative clause is:

• kraj koje je bila ona bijela jedrilica
• next to which that white sailboat was

If the subject were masculine, you would get je bio. If neuter, je bilo.

Ona is a demonstrative adjective, here meaning something like that.

So:

• ona bijela jedrilica = that white sailboat

It helps identify a specific sailboat, probably one already known from the conversation or visible in the situation being described.

Croatian often uses demonstratives where English might or might not use them, depending on context.

Croatian word order is fairly flexible, but this version is natural and clear.

Structure:

• Kad budemo opet na obali, = time clause first
• pokazat ću ti stijenu = main clause
• kraj koje je bila ona bijela jedrilica = relative clause describing stijenu

You could move parts around, but not completely freely, because clitics like ću, ti, and je have placement rules.

For example, this is also possible:

• Kad budemo opet na obali, pokazat ću ti onu stijenu kraj koje je bila bijela jedrilica.

But the original is very natural.

Because Kad budemo opet na obali is a subordinate clause introducing the time frame of the main clause.

Croatian normally separates this kind of introductory kad clause with a comma:

• Kad budemo opet na obali, pokazat ću ti...

This works much like English:

• When we’re back at the coast, I’ll show you...

In this sentence, yes, they are very close in meaning: again.

• opet = again
• ponovno = again / anew

Opet is extremely common in everyday speech and sounds perfectly natural here:

• Kad budemo opet na obali...

You could also say:

• Kad budemo ponovno na obali...

But opet is the more conversational and usual choice.

Not very naturally. A word-for-word version would be something like:

• When we-will-be again at the coast, I-will-show to-you the rock next-to which was that white sailboat.

Natural English would be:

• When we’re back at the coast, I’ll show you the rock next to which that white sailboat was.
• or more naturally still: When we’re back at the coast, I’ll show you the rock by the white sailboat.

So the Croatian grammar is doing several things differently from English:

• using future II after kad
• using cases instead of prepositions in the same way English does
• placing clitics like ću, ti, je according to Croatian word-order rules