Hvis det blir kjølig i kveld, drikker vi kakao i stedet for saft.

Yes. Hvis is the standard word for if when you are talking about a condition:

• Hvis det blir kjølig ... = If it gets cool ...

You may also see om in some kinds of if/whether sentences, but for a clear condition like this, hvis is the safest and most natural choice for learners.

Because blir means becomes / gets.

So:

• det er kjølig = it is cool
• det blir kjølig = it gets cool / it becomes cool

This sentence is about a change in the weather later in the evening, so blir fits better than er.

In Norwegian, det is often used as a dummy subject in weather expressions and other general statements, just like English it:

• Det er kaldt. = It is cold.
• Det blir mørkt. = It gets dark.

So det does not refer to a specific thing here. It is just the normal grammatical subject.

This is because of the Norwegian V2 rule: in a main clause, the finite verb usually comes in the second position.

The sentence starts with a subordinate clause:

• Hvis det blir kjølig i kveld

After that, the main clause begins, and the finite verb must come before the subject:

• drikker vi kakao ...

So the structure is:

• Hvis ... , drikker vi ...

If you started directly with the main clause, you would say:

• Vi drikker kakao i stedet for saft.

But once the Hvis clause comes first, inversion happens:

• Hvis ..., drikker vi ...

Norwegian often uses the present tense for future meaning when the time is already clear from the context.

Here, i kveld shows that this is about later today:

• Hvis det blir kjølig i kveld, drikker vi kakao ...

This is very natural in Norwegian. English often does something similar too:

• If it gets cold tonight, we’re drinking hot chocolate
• or more naturally: If it gets cold tonight, we’ll drink hot chocolate

Norwegian does not need a separate future form here.

Because adjectives ending in -ig or -lig usually do not add -t in the neuter form.

So:

• kjølig
• farlig
• hyggelig

all stay the same.

That is why Norwegian says:

• Det er kjølig
• Det blir kjølig

not kjøligt.

I kveld means this evening or tonight.

It is a time expression:

• i dag = today
• i morgen = tomorrow
• i kveld = tonight / this evening

In this sentence, it tells you when it might get cool.

I stedet for is a fixed expression meaning instead of.

So:

• kakao i stedet for saft = cocoa / hot chocolate instead of saft

You can use i stedet for with nouns, pronouns, or even larger phrases:

• te i stedet for kaffe = tea instead of coffee
• meg i stedet for ham = me instead of him

Because kakao and saft are being used as uncountable / mass nouns, just like English often says:

• We drink coffee
• We drink tea

Norwegian does the same:

• Vi drikker kakao
• Vi drikker saft

If you were talking about a specific serving, you might use something else, such as:

• en kopp kakao = a cup of cocoa
• et glass saft = a glass of saft

Not exactly.

Saft in Norwegian usually means a sweet fruit drink, often made from concentrate or syrup mixed with water. It is very common in Scandinavia.

So saft is sometimes translated as juice, but culturally it is often closer to:

• fruit squash
• cordial
• or a diluted sweet fruit drink

So the exact English word depends on context.

Because the sentence begins with a subordinate clause:

• Hvis det blir kjølig i kveld

In Norwegian, when a subordinate clause comes before the main clause, it is normally followed by a comma:

• Hvis ..., drikker vi ...

So the comma marks the boundary between the if-clause and the main clause.

In everyday Norwegian, kakao often refers to the drink that English speakers would commonly call hot chocolate or cocoa, depending on context.

So in a sentence like this, vi drikker kakao, the natural idea is usually a warm chocolate drink.

A very literal translation might be cocoa, but in natural English hot chocolate may sound more idiomatic in many situations.