Na pragu ostavi mokre cipele, a ne u hodniku.

Yes. This is an instruction or command addressed to one person.

The verb ostavi means leave in the 2nd person singular imperative, so the understood subject is you. Croatian usually does not need to say ti explicitly here.

So the structure is basically:

[You] leave the wet shoes on the threshold, not in the hallway.

If you wanted to address more than one person, you would say ostavite.

Ostavi is the imperative of ostaviti, which is a perfective verb.

That matters because perfective verbs often express a single completed action:

• ostavi = leave them / put them there as one action

Compare:

• ostavi = leave them there now, do it
• ostavljaj = keep leaving / leave habitually / leave repeatedly

So in this sentence, ostavi sounds natural because the speaker is telling someone to do one concrete thing.

Because na here means location, and with location it takes the locative case.

The noun is:

• prag = threshold, doorstep

Its locative singular is:

• pragu

So:

• na pragu = on the threshold / at the threshold

A useful pattern is:

• na stolu = on the table
• na podu = on the floor
• na pragu = on the threshold

If na meant movement toward a place, you might get a different case, but here it describes where the shoes should stay.

Because u can take different cases depending on meaning:

• u + locative = in / inside a place, showing location
• u + accusative = into a place, showing motion/destination

Here the phrase means location:

• u hodniku = in the hallway

So:

• u hodniku = in the hallway
• u hodnik = into the hallway

This is a very common Croatian pattern:

• u sobi = in the room
• u sobu = into the room

Because mokre must agree with cipele in gender, number, and case.

Here:

• cipele = shoes
• it is plural
• it is the direct object of ostavi, so it is in the accusative

Since cipele is a feminine plural noun, the adjective also takes the feminine plural accusative form:

• mokre cipele = wet shoes

For this noun, the accusative plural looks the same as the nominative plural, which is why it may seem unchanged.

It is plural.

• cipela = one shoe
• cipele = shoes

In everyday speech, footwear is very often talked about in the plural, since you normally mean the pair. So mokre cipele is a very natural phrase.

A ne creates a contrast. In this sentence it means something like:

• and not
• but not
• rather than

So the speaker is contrasting two places:

• na pragu
• u hodniku

The effect is: Leave the wet shoes on the threshold, not in the hallway.

Croatian often uses a for contrast, not only for simple and.

Because a is a coordinating conjunction, and here it introduces a contrasting phrase.

Croatian normally separates that kind of contrast with a comma:

• Na pragu ostavi mokre cipele, a ne u hodniku.

This is very similar to how English often uses punctuation to separate contrasting parts of a sentence.

Croatian word order is more flexible than English word order, so speakers can move parts of the sentence to change focus or emphasis.

Starting with Na pragu puts attention on the place:

• On the threshold, leave the wet shoes...

A more neutral order would also be possible:

• Ostavi mokre cipele na pragu, a ne u hodniku.

Both are correct, but the given sentence emphasizes where the shoes should go.

Because Croatian has no articles like English a and the.

So Croatian simply says:

• prag
• hodnik
• cipele

Whether English should translate them as a, the, or sometimes no article at all depends on context.

That means:

• na pragu can mean on the threshold / on a threshold
• u hodniku can mean in the hallway / in a hallway

In this sentence, English naturally uses the because the place is understood from the situation.

Yes, but it would be structured differently.

The original sentence gives a positive instruction and then a contrast:

• Na pragu ostavi mokre cipele, a ne u hodniku.

A version with nemoj would sound more directly negative, for example:

• Nemoj ostaviti mokre cipele u hodniku.
• Ostavi mokre cipele na pragu.

So:

• ostavi ... a ne ... = do X, not Y
• nemoj ... = don't do Y

Both are natural, but the original sentence is specifically built around contrast between two locations.