Vrati mi ključ i papir sutra.

Vrati is the imperative form, used to give a command or instruction. Here it is the 2nd person singular imperative, so it is speaking to one person informally: Return / bring back ...

It comes from the verb vratiti, which is a perfective verb.

Because vrati is used when speaking to one person informally.

• vrati = tell one person informally
• vratite = tell more than one person, or speak politely/formally to one person

So:

• Vrati mi ključ. = Return me the key. / Bring me back the key.
• Vratite mi ključ. = Return me the key. (formal or plural)

Mi means to me and is the dative singular form of ja.

It is a short unstressed form, often called a clitic.

So:

• mi = to me
• Vrati mi ključ = Return the key to me

Serbian often uses this short dative form where English uses to me.

Because mi is a clitic, and Serbian clitics usually appear very early in the sentence, typically in the second position area.

So Vrati mi ključ i papir sutra sounds natural because the command word vrati comes first, and the clitic mi follows early.

Learners often want to place it later, but Serbian strongly prefers short forms like mi, ti, ga, je, se near the beginning.

They are in the direct object case: accusative. But for masculine inanimate singular nouns, the accusative is usually the same as the nominative.

So:

• ključ = nominative singular

• ključ = accusative singular too

• papir = nominative singular
• papir = accusative singular too

That is why they look unchanged.

Compare with a masculine animate noun:

• student (nom.)
• studenta (acc.)

But with inanimate nouns like ključ and papir, nominative and accusative match.

Because Serbian does not have articles like English a/an/the.

So ključ can mean:

• a key
• the key

And papir can mean:

• a paper
• the paper

The exact meaning depends on context.

It can mean different things depending on context.

Common possibilities are:

• paper as a sheet of paper
• document / paper
• sometimes even official papers, depending on the situation

In this sentence, it most naturally means the paper / document / sheet of paper that should be returned.

Yes, sutra means tomorrow, and its position is somewhat flexible.

These are all possible, though the emphasis changes slightly:

• Vrati mi ključ i papir sutra.
• Sutra mi vrati ključ i papir.
• Vrati mi sutra ključ i papir.

Putting sutra at the end is perfectly natural. It often gives the sentence a simple, neutral flow.

Yes. Vratiti usually means to return, give back, or bring back something.

So the sentence implies that the person already has the key and paper, or should restore them to the speaker.

It is not just bring in a neutral sense; it has the idea of returning something.

Yes, aspect matters here.

Vratiti is perfective, so it presents the action as completed: return it successfully, as a single whole action.

That is very natural in commands when you want a concrete result.

An imperfective version would be based on vraćati. In the imperative, something like vraćaj would sound more like:

• keep returning
• be returning
• return habitually / repeatedly

So for a one-time command such as returning specific items, vrati is the normal choice.

Yes. Serbian, like English, can use one verb with two coordinated objects.

So:

• ključ i papir = the key and the paper

Both are direct objects of vrati.

You do not need to repeat the verb.

This sentence is natural, but there are several equally natural alternatives depending on tone and emphasis:

• Vrati mi ključ i papir sutra.
• Sutra mi vrati ključ i papir.
• Vrati mi sutra ključ i papir.

If you want to sound more polite, you could say:

• Vrati mi, molim te, ključ i papir sutra.
• Vratite mi ključ i papir sutra. (formal / plural)

So the original sentence is correct and natural, just on the direct side because it is a plain imperative.