Ako večeras zahladi, ponijet ću kaput.

It is a very common if-clause + main clause pattern:

• Ako večeras zahladi = If it gets cold tonight
• ponijet ću kaput = I’ll take a coat

So the whole sentence means:

• If it gets cold tonight, I’ll take a coat.

Croatian often uses this kind of structure very directly:

• ako = if
• condition first
• result second

Because after ako, Croatian normally uses the present tense form even when the meaning is future.

So:

• Ako večeras zahladi = If it gets cold tonight

not:

• Ako večeras će zahladiti ❌

This is very similar to English, which also often avoids will in if-clauses:

• If it gets cold tonight, I’ll take a coat. not
• If it will get cold tonight... ❌

So this part is actually quite natural for an English speaker once you notice the pattern.

Zahladi comes from the verb zahladiti and means it gets cold, it turns cold, or it becomes colder.

In this sentence it refers to the weather changing. Croatian often uses this kind of verb impersonally, without stating a subject.

English needs it:

• if it gets cold

Croatian does not need an equivalent subject word here:

• ako zahladi

So there is no missing noun; this is just a normal weather expression.

Because the sentence begins with a subordinate conditional clause:

• Ako večeras zahladi, = dependent clause
• ponijet ću kaput. = main clause

When the if-clause comes first, Croatian writes a comma before the main clause.

So the comma is not optional here in standard writing.

Večeras means tonight or this evening.

It is an adverb, so it can stand on its own without a preposition:

• večeras = tonight
• sutra = tomorrow
• danas = today

So Croatian says:

• Ako večeras zahladi...

just as English says:

• If it gets cold tonight...

No extra word like in is needed.

This is the future tense.

The full infinitive is ponijeti = to take along / to bring along.

Future I is made with:

• the infinitive
• plus the auxiliary ću

So the meaning is:

• ponijet ću = I will take

In standard Croatian spelling, when the infinitive comes before the auxiliary, the final -i of the infinitive drops:

• ponijeti + ću → ponijet ću

You can also say the same thing another way:

• Ja ću ponijeti kaput.

Both are correct.
The version in your sentence is a very normal, natural Croatian word order.

Ću is the 1st person singular future auxiliary, meaning I will.

The future auxiliary changes according to the subject:

• ću = I will
• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you will
• će = they will

So:

• ponijet ću kaput = I’ll take a coat

If the subject were different, the auxiliary would change:

• Ponijet će kaput = He/She will take a coat
• Ponijet ćemo kaput = We will take a coat

It is in the correct case: accusative, because it is the direct object of ponijeti.

The reason it looks unchanged is that masculine inanimate nouns often have the same form in the nominative and accusative singular.

So:

• nominative: kaput = a coat
• accusative: kaput = a coat as a direct object

That is why Croatian says:

• ponijet ću kaput

not because there is no case, but because this noun happens to look the same in both forms.

Ponijeti is a very natural choice here because it means something like:

• to take along
• to bring with you

That fits the idea of taking a coat in case it gets cold.

Compare:

• ponijeti kaput = take a coat along with you
• uzeti kaput = take a coat
• nositi kaput = wear/carry a coat, or wear coats in general depending on context

So ponijeti sounds especially good when you mean I’ll make sure to bring a coat with me.

Both verbs describe single complete events:

• zahladiti = to become cold / to turn cold
• ponijeti = to take along once

That is why the perfective aspect is natural here.

The sentence is not about:

• an ongoing process
• a habit
• repeated action

It is about one possible change in the weather and one response to it.

So the perfective aspect matches the meaning very well:

• If it turns cold tonight, I’ll take a coat.

Yes. Croatian word order is fairly flexible.

For example, you can also say:

• Ponijet ću kaput ako večeras zahladi.

This means the same thing.

The version with the if-clause first often sounds a bit more like:

• first the condition
• then the result

The version with the main clause first can sound slightly more conversational in some contexts.

However, the future auxiliary ću cannot normally stand alone as the very first unstressed word, so word order still follows clitic rules. That is why forms like ću ponijeti kaput by themselves are not normally used as neutral sentence openings.

Yes. That is also correct, but it is slightly different in feel.

• Ako večeras zahladi = If it gets cold tonight / if it turns cold
• Ako večeras bude hladno = If it is cold tonight

The first focuses on a change in temperature.
The second focuses more on the state of being cold.

Both are natural, but the original sentence suggests:

• it may become colder later
• and then I’ll take a coat