Ako valovi budu veliki, nećemo plivati blizu stijena.

In this sentence, budu is used because Croatian often uses future II in an if-clause when talking about a future situation.

• Ako valovi budu veliki... = If the waves are big / if the waves turn out to be big
• Main clause: nećemo plivati... = we will not swim...

This is a very common pattern:

• Ako bude kiše, ostat ćemo doma.
• Ako dođu rano, krenut ćemo odmah.

Why not the others?

• Ako su valovi veliki usually sounds more like a general or present-time statement: If the waves are big
• Ako će biti veliki is generally not the normal choice here in standard Croatian

So the pattern is:

• ako + future II
• main clause + future I

It is part of future II.

Future II is formed with:

• a form of biti: budem, budeš, bude, budemo, budete, budu
• plus either a past participle or, as here, a predicate expression

In this sentence:

• budu = the plural form
• veliki = the adjective complement

So budu veliki literally means something like be big in a future-dependent sense.

It does not mean a simple present tense.

Valovi is the nominative plural of val.

• singular: val = wave
• plural: valovi = waves

It is nominative because it is the subject of the clause:

• valovi budu veliki = the waves are / become big

So the sentence starts with the subject in the normal dictionary-based subject form.

Because adjectives in Croatian agree with the noun they describe in gender, number, and case.

Here:

• valovi is masculine plural nominative
• so the adjective must also be masculine plural nominative
• therefore: veliki

Compare:

• velik val = a big wave
• veliki valovi = big waves

Even though veliki comes after budu, it still agrees with valovi.

In standard Croatian, the negative future forms of htjeti are written as one word:

• neću
• nećeš
• neće
• nećemo
• nećete
• neće

So:

• nećemo plivati = we will not swim

This is an important spelling point. Learners often want to separate ne and the verb, but here they are written together.

Croatian future I is formed with:

• a future auxiliary, here nećemo
• plus the infinitive, here plivati

So:

• plivati = to swim
• nećemo plivati = we will not swim

This is a normal future construction.

Compare:

• Plivat ćemo. = We will swim.
• Nećemo plivati. = We will not swim.

Because Croatian usually does not need subject pronouns when the verb already shows the person and number.

• nećemo already means we will not
• so mi is unnecessary

You could say:

• Mi nećemo plivati blizu stijena.

But that would usually add emphasis, for example:

• We will not swim near the rocks.

In neutral sentences, Croatian usually omits the pronoun.

Blizu means near / close to, and it normally takes the genitive case.

That is why stijena is not nominative here.

The noun is:

• singular: stijena = rock, cliff
• genitive plural: stijena = of rocks / of cliffs

So:

• blizu stijena = near the rocks / close to the rocks

This is a useful pattern to remember:

• blizu kuće = near the house
• blizu grada = near the city
• blizu stijena = near the rocks

It can mean rock, large rock, or cliff, depending on context.

In a sentence about swimming and waves, stijena suggests rocky coastal rocks or cliffs by the sea. So English could translate it as:

• rocks
• the rocks
• sometimes cliffs, if the context is steeper coastal rock

The exact English word depends on the situation, but the Croatian form is perfectly natural.

Yes. Croatian word order is fairly flexible.

The original sentence:

• Ako valovi budu veliki, nećemo plivati blizu stijena.

You could also say:

• Nećemo plivati blizu stijena ako valovi budu veliki.

Both are correct. The first version puts the condition first, which is very natural and common.

Croatian often changes word order for emphasis, rhythm, or style, but the original sentence is a very standard neutral version.

Because the sentence begins with a subordinate if-clause:

• Ako valovi budu veliki, ...

When this clause comes first, it is normally separated by a comma from the main clause:

• Ako valovi budu veliki, nećemo plivati blizu stijena.

This works much like English:

• If the waves are big, we will not swim near the rocks.

If the main clause comes first, you may also see:

• Nećemo plivati blizu stijena ako valovi budu veliki.

In that order, a comma is usually not needed.

That would not be the normal standard choice here.

The standard pattern is:

• Ako + future II
• main clause + future I

So the correct sentence is:

• Ako valovi budu veliki, nećemo plivati blizu stijena.

Using ne budemo plivali would sound wrong in this context. For the main future statement, Croatian wants nećemo plivati.

The meaning is close, but the grammar is not identical.

English usually says:

• If the waves are big, we won’t swim near the rocks.

Notice that English uses a present form in the if-clause: are.

Croatian, however, often uses future II in that same type of future condition:

• Ako valovi budu veliki...

So a good learner rule is:

• English: if + present
• Croatian: often ako + future II

That difference is one of the most important things to notice in this sentence.