Ona mi vraća kusur posle večere.

Yes, you often can omit ona in Serbian if the subject is already clear from context, because the verb form vraća already suggests he/she/it returns.

So both are possible:

• Ona mi vraća kusur posle večere.
• Mi vraća kusur posle večere.

Including ona usually adds one of these effects:

• emphasis: She is the one returning the change
• contrast: not someone else, but she
• clarity: if it is important to identify the subject

Serbian often drops subject pronouns more than English does.

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

So:

• ja = I
• meni / mi = to me

In this sentence, mi is an unstressed clitic form, which is very common in everyday Serbian.

Compare:

• Ona mi vraća kusur. = She is giving me back the change.
• Ona vraća kusur meni. = She returns the change to me.
  This sounds more emphatic or contrastive.

Vraća is the 3rd person singular present tense of the verb vraćati / vratiti.

Here it agrees with ona:

• ja vraćam = I return
• ti vraćaš = you return
• on/ona/ono vraća = he/she/it returns

So ona vraća = she returns / she is returning

In Serbian, the present tense can often correspond to either:

• English simple present: she returns
• English present progressive: she is returning

The exact English translation depends on context.

Because vraća is a present tense form, while vrati is usually something else depending on context.

Most importantly:

• vraćati = imperfective, ongoing/repeated action
• vratiti = perfective, completed action

Examples:

• Ona mi vraća kusur. = She is returning / returns the change to me.
• Ona mi vrati kusur. can sound like a different construction, often not the normal neutral present meaning.

A key point for learners:
In Serbian, perfective verbs normally do not express a regular present action the same way English does. For ordinary present meaning, Serbian typically uses the imperfective verb, so vraća is the natural choice here.

Kusur means change in the money sense, like the money you get back after paying more than the price.

For example:

• You pay with a large bill.
• The cashier gives you back some money.
• That returned money is kusur.

It does not mean change in the sense of alteration or transformation.

Here kusur is in the accusative singular, but for this noun the accusative looks the same as the nominative.

That is very common with many masculine inanimate nouns in Serbian.

So:

• nominative: kusur
• accusative: kusur

It is the direct object of vraća — the thing being returned.

Because posle requires the genitive case.

The noun večera means dinner / supper, and its genitive singular form is večere.

So:

• nominative: večera
• genitive: večere

That is why you say:

• posle večere = after dinner

This is a very useful pattern to remember:

• posle ručka = after lunch
• posle posla = after work
• posle škole = after school

Yes. Serbian does not have articles like a or the, so večere can be understood from context as:

• after dinner
• after the dinner
• sometimes even after a dinner

In most everyday situations, English after dinner is the most natural translation.

The most natural order is Ona mi vraća kusur posle večere.

That is because mi is a clitic, and Serbian clitics usually go in a special early position in the sentence, often called the second position.

So this sounds natural:

• Ona mi vraća kusur posle večere.

This sounds unnatural or nonstandard in normal speech:

• Ona vraća mi kusur posle večere.

Serbian clitics like mi, ti, mu, je, se, sam, si, smo usually follow strict word-order rules, and this is one of the first things learners notice.

Yes. In natural English, return and give back can both work here.

Depending on context, these are all possible translations:

• She returns the change to me after dinner.
• She gives me back the change after dinner.
• She is giving me back the change after dinner.

In everyday English, gives me back the change often sounds more natural than returns the change to me, even though the Serbian verb is literally related to returning.

It can mean either dinner or supper, depending on the speaker, region, and eating habits.

So posle večere may be understood as:

• after dinner
• after supper

In many learning materials, dinner is the default translation, but supper may also be correct in context.

Yes, the grammar is completely natural.

That said, depending on context, a speaker might also choose slightly different wording, for example:

• Ona mi vraća kusur posle večere.
• Posle večere mi vraća kusur.
• Vraća mi kusur posle večere.

The original sentence is a normal, correct Serbian sentence and is perfectly suitable for learning several useful features:

• dropped or expressed subject pronouns
• dative clitic mi
• present tense vraća
• direct object kusur
• posle + genitive → posle večere