Nemoj ostaviti kofer na peronu.

This is the normal way to make a negative command in Croatian when speaking to one person informally.

• nemoj ostaviti = don’t leave
• literally, nemoj comes from the verb moći / moći? No—in this construction it functions as a fixed imperative form meaning don’t.

So:

• Ostavi kofer. = Leave the suitcase.
• Nemoj ostaviti kofer. = Don’t leave the suitcase.

This pattern is extremely common in Croatian.

This is about aspect.

• ostaviti is perfective
• ostavljati is imperfective

In commands, Croatian often uses the perfective verb when talking about a single completed action.

So here:

• Nemoj ostaviti kofer na peronu. means Don’t leave the suitcase on the platform as a one-time act.

If you used nemoj ostavljati, it would sound more like:

• Don’t be leaving the suitcase there
• Don’t leave it there repeatedly / habitually

So ostaviti is the natural choice for a simple warning about one action.

Because kofer is the direct object of the verb ostaviti, so it goes in the accusative case.

For many masculine inanimate nouns in Croatian, the accusative singular is the same as the nominative singular.

So:

• nominative: kofer
• accusative: kofer

That is why the form does not change.

Because na here means on or at in a location sense, not movement toward a destination.

When na expresses location, it takes the locative case.

• peron = platform
• locative singular: peronu

So:

• na peronu = on the platform

Compare:

• Stani na peron. = Step onto the platform. → movement, so accusative
• Kofer je na peronu. = The suitcase is on the platform. → location, so locative

Peron usually means a station platform, especially at a train or bus station.

So na peronu is typically:

• on the platform
• at the platform

A learner may notice that Croatian also has words like platforma, but peron is the usual everyday word in this transport context.

It is informal singular—used when speaking to one person in a casual or familiar way.

Related forms are:

• Nemoj ostaviti... = informal singular
• Nemojte ostaviti... = plural or formal singular
• Nemojmo ostaviti... = let’s not leave...

So if you were speaking politely to one person, you would usually say:

• Nemojte ostaviti kofer na peronu.

In standard everyday Croatian, nemoj + infinitive is the usual and most natural way to make a negative imperative.

So:

• natural: Nemoj ostaviti kofer na peronu.
• much less usual / not the standard pattern for ordinary speech: Ne ostavi kofer na peronu.

As a learner, it is safest to remember:

• positive command: imperative form
  
  • Ostavi kofer.
• negative command: nemoj + infinitive
  
  • Nemoj ostaviti kofer.

Yes, Croatian word order is fairly flexible, although some orders sound more natural in neutral speech.

The given sentence is a normal neutral order:

• Nemoj ostaviti kofer na peronu.

But you could also hear:

• Kofer nemoj ostaviti na peronu.
  This puts more emphasis on the suitcase.
• Nemoj na peronu ostaviti kofer.
  This puts more emphasis on the platform.

Even when the order changes, the cases show the grammatical roles clearly.

It is pronounced roughly os-ta-vi-ti.

The v is fully pronounced. The verb breaks down like this:

• o-stav-i-ti

A rough English-friendly approximation would be:

• AW-stah-vee-tee

But Croatian pronunciation is more regular than English spelling, so each vowel is pronounced clearly.

Yes. It helps to learn Croatian verbs in aspect pairs.

Here the common pair is:

• ostavljati = imperfective
• ostaviti = perfective

Very roughly:

• ostavljati focuses on process, repetition, or habit
• ostaviti focuses on a completed result

Examples:

• Ostavlja kofer svaki dan tamo. = He leaves the suitcase there every day.
• Ostavi kofer tamo. = Leave the suitcase there.
• Nemoj ostaviti kofer tamo. = Don’t leave the suitcase there.

Learning the pair together makes Croatian much easier.

Yes, it could. It sounds like a direct warning or instruction.

For a public sign or official announcement, Croatian might also use slightly different wording depending on style, for example:

• Ne ostavljajte kofere na peronu. = Do not leave suitcases on the platform.
• Nemojte ostavljati prtljagu na peronu. = Do not leave luggage on the platform.

But the original sentence is perfectly natural as something one person says to another.