Μην βάλεις πολύ αέρα στο μπαλόνι, γιατί θα σκάσει.

Μην is the word used for a negative command, so here it means don’t.

Greek does not normally use δεν for commands.

• δεν = negates statements
• μη(ν) = negates commands and subjunctive-type forms

So:

• Βάλε... = Put...
• Μην βάλεις... = Don’t put...

In meaning, yes: it is giving a command to you. But grammatically, Modern Greek usually forms negative commands with μην + subjunctive/dependent form, not with a separate negative imperative form.

So:

• positive command: Βάλε πολύ αέρα...
• negative command: Μην βάλεις πολύ αέρα...

That is why you see μην βάλεις instead of a special “negative imperative” ending.

Because Greek is making the action perfective: a single complete act of putting air in.

• μην βάλεις = don’t put (one complete act)
• μην βάζεις = don’t keep putting / don’t be putting / don’t put habitually

With a balloon, the speaker is warning about one specific act: adding too much air. So βάλεις is the natural choice.

This is very common in Greek. Many verbs have:

• a present/imperfective stem
• a perfective/aorist stem

For βάζω:

• present stem: βαζ-
• perfective stem: βαλ-

So you get forms like:

• βάζω = I put / I am putting
• έβαλα = I put (past, completed)
• να βάλω = to put / that I put
• μην βάλεις = don’t put

So yes, it looks irregular, but it follows a common Greek pattern.

Because Greek often leaves subject pronouns out. The verb ending already tells you who the subject is.

Here βάλεις shows second person singular, so the meaning is clearly you.

You could say Μην βάλεις εσύ... if you want extra emphasis, but normally εσύ is unnecessary.

Because αέρας here means air as a mass noun, and Greek uses πολύ with it in this structure.

Also, πολύς is the dictionary form, but the masculine accusative singular form is πολύ, which matches αέρα.

So:

• ο πολύς αέρας = the lot of air / much air (subject form)
• βάζω πολύ αέρα = I put a lot of air (object form)

English usually translates this as too much air or a lot of air depending on context.

Because it is the direct object of the verb βάλεις.

The dictionary form is:

• ο αέρας = air

But the accusative singular is:

• τον αέρα

In Greek, masculine nouns like this often lose the final -ς in the accusative. So after βάλεις, you get αέρα.

στο is a contraction of σε + το.

• σε can mean to, in, on, into, at
• το = the (neuter singular)

So στο μπαλόνι literally means something like in/into the balloon.

In this sentence, English would most naturally say into the balloon.

It is accusative singular after σε.

However, μπαλόνι is a neuter noun, and in many neuter nouns the nominative and accusative singular look exactly the same.

So even though the form is still μπαλόνι, grammatically it is accusative here:

• το μπαλόνι = the balloon
• στο μπαλόνι = into/in the balloon

Here γιατί means because.

That same word can also mean why in a question:

• Γιατί έφυγες; = Why did you leave?

But in this sentence it introduces the reason:

• ..., γιατί θα σκάσει.
• ..., because it will burst/pop.

θα is the future particle, and σκάσει is the perfective dependent form of σκάω.

Together, θα σκάσει means:

• it will burst
• it will pop

So Greek forms the future here with:

• θα + verb form

This is the normal way to make the future in Modern Greek.

Because the speaker means one complete future event: the balloon will burst once.

• θα σκάσει = it will burst/pop
• θα σκάει would be imperfective and is not the natural choice here

Since a balloon popping is a single complete event, σκάσει is exactly what Greek expects.

For a balloon, σκάσει means burst, pop, or sometimes explode in an everyday, non-technical sense.

The most natural English choice is often pop:

• The balloon will pop.

So this is a very ordinary word to use with balloons, tires, bubbles, and similar things.

Yes. Greek word order is fairly flexible, although the given version is the most neutral and natural.

For example, you might also hear:

• Μην βάλεις στο μπαλόνι πολύ αέρα.

That still means the same thing, but the emphasis shifts a little. The original order is a very normal way to say it.