Ako se sutra otopli, pijemo kavu na terasi.

Because in Croatian, after ako (if), you normally do not use the future tense marker će the way English often uses will.

So Croatian says:

• Ako se sutra otopli...
• literally: If it gets warmer tomorrow...

not:

• Ako će se sutra otopliti... ← this sounds wrong in standard Croatian for this kind of sentence

This is very common in Croatian: the verb in the if-clause is often in the present form, even when the meaning is future.

Se is a very common Croatian particle used with many verbs. In this sentence, it is part of the verb otopliti se, which means to get warmer / to warm up.

So:

• otopliti = to melt / to make warmer in some contexts
• otopliti se = to get warmer, especially about the weather

Here it does not really mean it warms itself in a literal English sense. It is just how the verb works in Croatian.

This kind of se is often hard for English speakers because English usually does not mark this idea the same way.

Because this is a normal Croatian pattern.

The form otopli is a present-tense form of a perfective verb, and perfective present forms often refer to a future completed change:

• Ako se sutra otopli = If it gets warmer tomorrow

So grammatically it looks like present, but in meaning it points to a future event.

This is especially common in subordinate clauses such as:

• kad dođe = when he comes
• ako stigne = if he arrives / if he manages to get here
• čim završi = as soon as he finishes

Croatian often uses the present tense to talk about a future action when it is seen as a plan, arrangement, or something that will happen if the condition is met.

So:

• Ako se sutra otopli, pijemo kavu na terasi.

means something like:

• If it gets warmer tomorrow, we’re having coffee on the terrace.

This is very similar to English We’re having coffee on the terrace tomorrow rather than We will drink coffee.

You could also say:

• Ako se sutra otopli, pit ćemo kavu na terasi.

That is also correct, but it sounds a bit more like a straightforward future statement.
Pijemo feels a little more immediate, natural, and plan-like.

Because Croatian usually drops subject pronouns when they are not needed.

The verb form pijemo already tells you the subject is we, because the ending -mo marks 1st person plural.

So:

• pijemo = we drink / we are drinking / we will drink depending on context

You only add mi if you want emphasis or contrast:

• Mi pijemo kavu na terasi, a oni ostaju unutra.
• We are having coffee on the terrace, and they are staying inside.

Because kavu is the accusative singular form of kava (coffee).

The verb piti (to drink) takes a direct object, and direct objects are usually in the accusative.

So:

• kava = nominative, dictionary form
• kavu = accusative, used after piti

Examples:

• Kava je dobra. = The coffee is good.
• Pijem kavu. = I’m drinking coffee.

Because na can take different cases depending on the meaning.

Here, na terasi means on the terrace / at the terrace as a location, so Croatian uses the locative:

• na terasi = on the terrace

If there were movement toward the terrace, Croatian would usually use the accusative:

• Idemo na terasu. = We’re going onto / to the terrace.

So the contrast is:

• na terasi = location
• na terasu = destination

Croatian word order is fairly flexible.

The sentence:

• Ako se sutra otopli, pijemo kavu na terasi.

is natural, but sutra can move:

• Ako sutra se otopli... ← possible in theory, but less natural because se tends to want second position
• Ako se otopli sutra, pijemo kavu na terasi. ← possible, but usually less natural unless you want to emphasize tomorrow
• Pijemo sutra kavu na terasi ako se otopli. ← also possible in conversation

The original version sounds smooth and neutral.

A useful thing to remember is that se often prefers an early position in the clause, which affects word order.

Because the sentence begins with a subordinate if-clause:

• Ako se sutra otopli = subordinate clause
• pijemo kavu na terasi = main clause

In Croatian, it is normal to separate these with a comma:

• Ako se sutra otopli, pijemo kavu na terasi.

This is similar to English:

• If it gets warmer tomorrow, we’re having coffee on the terrace.

Croatian verbs have aspect, which is a very important idea for English speakers to learn.

In this sentence:

• otopliti se is perfective
• piti is imperfective

Why?

1. otopliti se is perfective

It describes a change of state as a completed event:

• if it gets warmer

The focus is on the weather becoming warmer.

2. piti is imperfective

It describes the activity of drinking/having coffee as an event or situation, not as a single completed result.

That is why pijemo kavu sounds natural here.

If you used popijemo kavu, that would suggest more of a completed act of drinking up the coffee, which is possible in some contexts but not the most neutral phrasing here.

It can definitely mean we have coffee, which is often the most natural English translation here.

In Croatian, piti kavu is the normal expression for:

• drinking coffee
• having coffee
• sitting down for coffee socially

So in this sentence, it does not just describe the physical act of swallowing coffee. It can also suggest the whole social activity:

• If it gets warmer tomorrow, we’re having coffee on the terrace.

That is probably the best way to understand it.

Yes. That is another very natural way to express a similar idea.

Compare:

• Ako se sutra otopli... = If it gets warmer tomorrow...
• Ako sutra bude toplije... = If it is warmer tomorrow...

The difference is small:

• otopli se emphasizes the change in weather
• bude toplije emphasizes the resulting condition of being warmer

Both are common and natural, but your original sentence is perfectly good Croatian.

It is neutral and natural. It works well in everyday speech and writing.

Nothing in it is especially slangy, bookish, or overly formal. It sounds like a normal conversational sentence a Croatian speaker might say when making plans based on the weather.

So it is a very useful model sentence to learn from.