Ako nemaš torbu, uzmi papirnatu vrećicu.

Because the negative form of imati (to have) is normally written as one word in the present tense:

• nemam = I don't have
• nemaš = you don't have
• nema = he/she/it doesn't have

So nemaš is just the standard form for you don't have.

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

Here, nemaš clearly means you don't have, and uzmi is also a command addressed to you (singular). Because of that, ti is not necessary.

You can add ti for emphasis or contrast, but in a neutral sentence it is usually omitted.

They are in the accusative singular, because they are the direct objects of the verbs:

• nemaš torbu = you don't have a bag
• uzmi vrećicu = take a bag

Both torba and vrećica are feminine nouns, and feminine nouns of this type usually change -a to -u in the accusative singular:

• torba → torbu
• vrećica → vrećicu

Because adjectives in Croatian must agree with the noun they describe in gender, number, and case.

The noun vrećicu is:

• feminine
• singular
• accusative

So the adjective must match it:

• papirnata vrećica = nominative
• papirnatu vrećicu = accusative

So papirnatu is simply the correct form to go with vrećicu.

Both can translate as bag, but they are not quite the same.

• torba is a more general word for a bag: handbag, tote bag, school bag, etc.
• vrećica usually means a lighter, simpler bag, especially a shopping bag or carrier bag.

In this sentence, papirnata vrećica means a paper bag of the kind you might take in a shop.

Also, vrećica is historically a diminutive form, but in everyday Croatian it is very often just the normal word for a small/store bag.

Uzmi is the 2nd person singular imperative, meaning it is a command to one person: take.

It comes from the verb uzeti (to take, perfective).

So:

• uzmi = take! (one person)
• uzmite = take! (more than one person, or formal singular)

Because uzmi is the natural form for a single, complete action: take a paper bag.

• uzimaš means you take / you are taking — that is not a command
• uzimaj is also a command, but from the imperfective verb uzimati, and it can sound more like repeated, ongoing, or less bounded action

In a practical instruction like this, uzmi is the normal choice.

Because Ako nemaš torbu is an introductory if-clause (a subordinate clause), and Croatian normally separates that clause from the main clause with a comma when it comes first:

• Ako nemaš torbu, uzmi papirnatu vrećicu.

So the comma marks the boundary between the condition and the main instruction.

Because Croatian commonly uses the present tense in real, everyday conditions:

• Ako nemaš torbu, uzmi papirnatu vrećicu.

This is the normal way to say If you don't have a bag, take a paper bag.

It is not talking about an unreal or hypothetical situation. It is just a practical condition: if this is the case, then do this.

Yes. Croatian word order is fairly flexible. You could also say:

• Uzmi papirnatu vrećicu ako nemaš torbu.

The basic meaning stays the same, but the emphasis shifts slightly:

• Ako nemaš torbu... puts the condition first
• Uzmi papirnatu vrećicu... puts the instruction first

The version in your sentence sounds very natural because it first states the condition and then gives the solution.