Okupićemo se rano da bismo naduvali balone.

It is the future tense of okupiti se.

In standard Serbian, when the short future auxiliary (ću, ćeš, će, ćemo, ćete) comes after an infinitive ending in -ti, it is usually written together with the verb stem:

• okupiti + ćemo → okupićemo

So okupićemo se means we will gather / we will get together.

You can also put the auxiliary before the infinitive, in which case it stays separate:

• Mi ćemo se okupiti.

Both are normal, but the sentence you were given uses the joined form.

Se is the reflexive particle, and it is part of the verb okupiti se.

• okupiti = to gather something/someone
• okupiti se = to gather, to get together, to assemble

So:

• Okupićemo decu. = We will gather the children.
• Okupićemo se. = We will gather / get together.

In English, we usually do not say gather ourselves, but Serbian often uses this reflexive structure where English does not.

Rano is an adverb, meaning early.

It modifies the verb phrase okupićemo se, so it tells you when the gathering will happen:

• Okupićemo se rano = We’ll gather early

The same form rano can also appear as a neuter adjective in other contexts, but here it is clearly an adverb.

Here da bismo introduces a purpose clause. In English, this is usually translated as:

• so that
• in order to

So:

• da bismo naduvali balone = so that we could blow up the balloons / in order to blow up the balloons

A useful way to think about it is:

• da = that / so that
• bismo = we would

But in natural English, you usually would not translate it word for word. The whole phrase works together to express purpose.

Because Serbian normally uses da + conditional to express purpose in this kind of sentence.

• da bismo naduvali balone = so that we could / in order to blow up the balloons

If you say:

• da ćemo naduvati balone

that usually means that we will blow up the balloons, not in order to blow up the balloons. In other words, it sounds like a statement about the future, not a purpose.

So in this sentence, da bismo is the right structure.

Because it is the l-participle form of the verb naduvati, and Serbian uses that form in more than one construction.

You see similar-looking forms in the past tense:

• Mi smo naduvali balone. = We blew up the balloons.

But you also use the same participle in the conditional:

• Mi bismo naduvali balone. = We would blow up the balloons.

So in da bismo naduvali, the form naduvali is not functioning as a past tense by itself. It is part of the conditional-style structure used here for purpose.

Because naduvati means to inflate / blow up fully, while duvati means more generally to blow.

Compare:

• duvati = to blow
• naduvati = to blow up, inflate

In this sentence, the goal is a completed result: the balloons end up inflated. That is why naduvati is the natural choice.

This is also about aspect:

• naduvati is perfective: it focuses on completing the action
• naduvavati would be imperfective: repeated, ongoing, or habitual inflating

Since the sentence expresses a specific goal, the perfective verb fits well.

Because balone is the accusative plural, and the balloons are the direct object of naduvati.

The noun is:

• singular nominative: balon
• plural nominative: baloni
• plural accusative: balone

So:

• Baloni su veliki. = The balloons are big.
  Here baloni is the subject.
• Naduvali smo balone. = We blew up the balloons.
  Here balone is the object.

English does not show this kind of noun case ending, but Serbian does.

Yes, often it could.

• Okupićemo se rano da naduvamo balone.

is also a natural Serbian sentence.

The version with da bismo sounds a bit more explicitly purposive, and sometimes a bit more formal or carefully structured:

• da naduvamo = to blow up / so we blow up
• da bismo naduvali = so that we could / in order to blow up

In many everyday contexts, both are possible. The version with da bismo simply highlights the idea of purpose a little more clearly.

No, Serbian word order is fairly flexible, although some orders sound more neutral than others.

The given sentence:

• Okupićemo se rano da bismo naduvali balone.

is a very natural, neutral way to say it.

You could also hear:

• Rano ćemo se okupiti da bismo naduvali balone.

This means the same thing, but the emphasis shifts slightly toward rano.

So the exact order can change, but you still have to respect normal Serbian placement patterns, especially for short words like se and auxiliary forms such as ćemo and bismo.