Μην πετάς το καλαμάκι στο δρόμο· βάλ’ το στον κάδο.

Because Greek uses μη(ν) for negative commands.

• Μην πετάς = Don’t throw
• Δεν πετάς = You don’t throw / You are not throwing

So δεν negates statements, while μην is used to tell someone not to do something.

It is speaking to one person, informally.

• πετάς = you throw / don’t throw for one person
• βάλ’ comes from βάλε, which is also 2nd person singular

If you were speaking to more than one person, or using the polite plural, you would say something like:

• Μην πετάτε το καλαμάκι στο δρόμο· βάλτε το στον κάδο.

Βάλ’ το is a shortened form of βάλε το.

In Greek, the final unstressed -ε of some imperative forms is often dropped before a weak pronoun:

• βάλε το → βάλ’ το
• πάρε το → πάρ’ το
• δώσε μου → δώσ’ μου

The apostrophe shows that a sound has been omitted.

The two το forms do different jobs.

• το καλαμάκι: here το is the definite article = the
• βάλ’ το: here το is the object pronoun = it

So the sentence is literally structured like:

• Don’t throw the straw in the street; put it in the bin.

This is about the final -ν.

Both δρόμο and κάδο are masculine accusative singular forms:

• ο δρόμος → τον δρόμο
• ο κάδος → τον κάδο

After σε, this becomes:

• σε τον δρόμο → στον δρόμο
• σε τον κάδο → στον κάδο

But in modern Greek, the final -ν is often dropped before certain consonants. So στον δρόμο is very often written and said as στο δρόμο. Before κ, the -ν is normally kept, so στον κάδο.

So the difference is not one of meaning; it is just a matter of pronunciation/spelling.

They are both in the accusative singular.

Greek uses σε + accusative for both:

• location: in/on/at
• direction: to/into

So:

• στο δρόμο = in/on the street
• στον κάδο = in/into the bin

This is normal Greek usage after σε.

Because with an affirmative imperative, weak object pronouns usually come after the verb.

• βάλ’ το = put it
• πάρ’ το = take it

But with most other verb forms, the pronoun usually comes before:

• το βάζω = I put it
• θα το βάλω = I will put it

This creates a nice contrast with negative commands. Greek negative commands use μη(ν) + verb, and there the pronoun normally comes before the verb:

• Μην το πετάς = Don’t throw it

-άκι is a very common Greek diminutive ending.

It often suggests:

• something small
• something little
• sometimes simply the normal everyday name of an object

So καλαμάκι originally means something like little reed, which is why it came to mean straw.

Greek uses diminutives a lot, so this is a very useful ending to recognize.

This is the Greek άνω τελεία.

It works roughly like a semicolon or a strong pause between two related parts of a sentence:

• Μην πετάς το καλαμάκι στο δρόμο· βάλ’ το στον κάδο.

Important: it is not the Greek question mark.

The Greek question mark looks like this:

• ;

So that is a common thing for learners to watch out for.

Both can be possible, but they express different aspect.

• μην πετάς = imperfective
• μην πετάξεις = perfective

Very roughly:

• μην πετάς can sound more like don’t go throwing it, don’t throw it around, or a more general instruction
• μην πετάξεις sounds more like don’t throw it in this specific one-time event

English usually just says don’t throw, but Greek often chooses between these two aspects more clearly.

Again, this is an aspect difference.

• βάλε / βάλ’ το = perfective imperative, for a single complete action
• βάζε το = imperfective imperative, for a repeated, habitual, or ongoing action

Here the speaker wants one simple action:

• Put it in the bin.

So βάλ’ το is the natural choice.