Ako se do podneva ne razvedri i ne otopli, nećemo ići na obalu.

This is a very common pattern in Croatian.

After ako (if), Croatian usually uses a present-tense form in the subordinate clause, even when the meaning is future:

• Ako se razvedri... = If it clears up...
• Ako otopli... = If it gets warmer...

Then the main clause shows the future more explicitly:

• nećemo ići = we will not go

So the structure is:

• Ako + present, future in the main clause

This is more natural in Croatian than using a future form after ako.

Here, se is part of the verb razvedriti se, which means to clear up when talking about weather or the sky.

So:

• razvedriti can have other meanings in other contexts
• razvedriti se is the usual form for weather becoming clear

In this sentence, you should learn se razvedri as a set expression meaning something like:

• it clears up
• the weather clears
• the sky clears

English uses it, but Croatian does not need a subject here.

Because Croatian often uses impersonal verbs for weather.

In English, you say:

• it clears up
• it gets warmer
• it rains

But in Croatian, no dummy subject like it is needed. The verb simply appears in 3rd person singular:

• razvedri
• otopli

This is completely normal with weather expressions.

In Croatian, when two verbs are joined by i (and), each verb is usually negated separately if both are negative.

So Croatian says:

• ne razvedri i ne otopli

literally:

• does not clear up and does not get warmer

This is more natural than putting one ne and letting it cover both verbs.

Do podneva means by noon or until noon.

The preposition do takes the genitive case, and podneva is the genitive singular form of podne (noon).

So:

• podne = noon
• do podneva = by/until noon

This is a very common pattern:

• do jutra = by morning
• do večeri = by evening
• do kraja = by the end / until the end

Croatian future tense is often formed with:

• a form of htjeti (to want) used as an auxiliary
• plus the infinitive

So:

• ćemo ići = we will go
• nećemo ići = we will not go

In the negative, the auxiliary and ne are written together:

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

So nećemo ići is the normal way to say we won’t go.

Both ideas are related, but the word order changes depending on whether the future is affirmative or negative.

In affirmative sentences, Croatian often places the infinitive first and the auxiliary second:

• ići ćemo = we will go

But in negative sentences, the auxiliary carries the negation and normally comes before the infinitive:

• nećemo ići = we will not go

So:

• ići ćemo = affirmative
• nećemo ići = negative

That is the normal standard pattern.

Na obalu uses the preposition na with the accusative case because it expresses movement toward a destination.

So:

• obala = coast, shore
• obalu = accusative singular
• na obalu = to the coast / to the shore

A very useful contrast is:

• na obalu = toward the coast
• na obali = on/at the coast

So in this sentence, because the meaning is go to the coast, the accusative is correct.

Not exactly in the narrow sense.

Obala usually means:

• coast
• shore
• waterfront
• sometimes riverbank, depending on context

In this sentence, ići na obalu most naturally means:

• go to the coast
• go to the seaside

If Croatian wanted to say specifically beach, it would often use plaža.

So obala is a little broader than English beach.

Croatian often omits subject pronouns because the verb ending already shows the person and number.

Here:

• nećemo ends in -mo, which tells you the subject is we

So mi is understood automatically.

Croatian usually includes mi only when it wants to add emphasis or contrast:

• Mi nećemo ići na obalu. = We won’t go to the coast.
  (perhaps implying someone else might)

Without emphasis, leaving out mi is more natural.

They are 3rd person singular present-tense forms of perfective verbs:

• razvedriti se
• otopliti

A useful thing to know is that perfective present in Croatian often does not describe something happening right now. Instead, it often refers to a future, completed event, especially in clauses with words like:

• ako = if
• kad = when

So here:

• ako se razvedri = if it clears up
• ako otopli = if it gets warmer

This is one reason the sentence sounds future-oriented even though those verbs are formally present tense.

Because the sentence begins with a subordinate conditional clause:

• Ako se do podneva ne razvedri i ne otopli = If it doesn’t clear up and get warmer by noon

Then comes the main clause:

• nećemo ići na obalu = we won’t go to the coast

In Croatian, when a subordinate clause comes first, it is normally separated from the main clause by a comma.

So the comma here is standard and expected.