Naduvaćemo balone.

Because Serbian can form the future in two ways:

• Mi ćemo naduvati balone.
• Naduvaćemo balone.

In the second pattern, the infinitive naduvati loses -ti, and the future auxiliary ćemo attaches to it:

naduvati + ćemo → naduvaćemo

Both are correct.

The dictionary form is naduvati.

A useful comparison is:

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

So the prefix na- helps turn the basic idea of blowing into inflating.

Because Serbian often leaves subject pronouns out when they are already clear from the verb form.

Here, -ćemo already tells you the subject is we, so mi is unnecessary unless you want emphasis:

• Naduvaćemo balone. = neutral
• Mi ćemo naduvati balone. = we will inflate the balloons

It is perfective. That means it presents the action as a completed whole: the balloons will end up inflated.

That is very natural here, because the sentence describes a future action with a clear result.

An imperfective counterpart is naduvavati, but the perfective naduvati is the normal choice for a one-time completed action like this.

Because balone is the accusative plural, used for the direct object of the verb.

Here, the balloons are what will be inflated, so they are the object:

• baloni = nominative plural, used for the subject
• balone = accusative plural, used for the object

Compare:

• Baloni su veliki. = The balloons are big.
• Naduvaćemo balone. = We will inflate the balloons.

It can mean either, depending on context.

Serbian does not have articles like a/an and the, so definiteness is usually understood from the situation.

So Naduvaćemo balone can mean:

• We’ll inflate balloons
• We’ll inflate the balloons

The context tells you which is meant.

Yes. That is completely correct.

Serbian word order is fairly flexible, so different versions can sound natural:

• Naduvaćemo balone. = neutral, compact
• Mi ćemo naduvati balone. = emphasizes we
• Balone ćemo naduvati. = emphasizes balloons

The basic meaning stays the same.

Ć is a soft consonant, somewhat softer than English ch.

A rough approximation of naduvaćemo is:

na-doo-VA-che-mo

But the ch there should be softer than in chair. If you pronounce it a bit like a soft ty/ch sound, you will be close enough for a learner.