Posle škole ću ti vratiti olovku.

Because posle means after and it requires the genitive case.

• dictionary form: škola = school
• after posle: posle škole = after school

So škole is the genitive singular form of škola.

Because olovku is the direct object of the verb vratiti (to return), so it goes in the accusative case.

• nominative: olovka = a pencil / the pencil
• accusative: olovku

So:

• olovka = subject form
• olovku = object form

In this sentence, the pencil is the thing being returned, so accusative is needed.

Ću is the 1st person singular future-tense auxiliary, meaning roughly I will.

In Serbian, the future is often made with:

• a form of hteti as a clitic auxiliary
• plus the infinitive

So:

• ću vratiti = I will return

Because ću already tells you the subject is I, Serbian does not need to say ja unless you want emphasis.

Yes. In Serbian, future forms like this can appear in two common ways:

• ću vratiti
• vratiću

Both mean I will return.

In your sentence, the version with the separated auxiliary is used:

• Posle škole ću ti vratiti olovku.

The joined form is also very common:

• Posle škole vratiću ti olovku.

Both are natural. Learners should recognize both patterns.

Ti here means to you.

It is the dative singular clitic form of ti (you, singular/informal).

So:

• vratiti olovku = to return the pencil
• vratiti ti olovku = to return the pencil to you

This ti is not the stressed subject pronoun you. It is an unstressed short form used as an indirect object.

Because Serbian often drops subject pronouns when the verb already makes the subject clear.

Here, ću already tells us the subject is I.

So:

• (Ja) ću ti vratiti olovku.
• Ću ti vratiti olovku. would not normally stand alone because clitics usually need something before them
• Posle škole ću ti vratiti olovku. is perfectly normal

You could add ja for emphasis:

• Ja ću ti vratiti olovku. = I will return the pencil to you / I’m the one who will return it

Serbian word order is flexible, but clitics like ću and ti usually have to appear near the beginning of the clause, often in second position.

That is why this order sounds natural:

• Posle škole ću ti vratiti olovku.

Here:

• Posle škole is the first phrase
• then come the clitics ću ti
• then the main verb and object

Other word orders are possible, for example:

• Olovku ću ti vratiti posle škole.
• Ja ću ti vratiti olovku posle škole.

But the placement of ću and ti is not random.

Vratiti is a perfective verb. It means to return something / give something back, viewed as a completed action.

That makes it very natural in the future here, because the speaker is promising one completed act:

• I’ll give the pencil back

Its imperfective partner is usually vraćati:

• vratiti = to return once / complete the return
• vraćati = to be returning / return repeatedly / habitually

So in this sentence, vratiti is the right choice.

No.

• vratiti = to return something
• vratiti se = to return oneself / come back

Examples:

• Vratiću ti olovku. = I’ll return the pencil to you.
• Vratiću se posle škole. = I’ll come back after school.

In your sentence there is no se, because the speaker is returning an object, not coming back themselves.

In this sentence it means after school, in the everyday time-related sense.

Serbian has no articles like a or the, so škole does not literally show whether English should use the or not. You decide from context.

Here, the natural English meaning is:

• after school

not

• after the school

Yes. Nakon škole also means after school.

Both are correct:

• Posle škole ću ti vratiti olovku.
• Nakon škole ću ti vratiti olovku.

Posle is extremely common in everyday speech. Nakon can sound a little more formal or neutral depending on context, but both are standard.

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

So:

• olovka / olovku can mean a pencil or the pencil
• škola / škole can mean school or the school

The exact English translation depends on context. In this sentence, English naturally uses:

• after school
• the pencil

Not by itself. The sentence only explicitly says:

• I’ll return the pencil to you after school

It does not directly say whose pencil it is. The ownership might be understood from context, but grammatically olovku just means the pencil / a pencil.

If you wanted to say your pencil, Serbian would normally make that explicit, for example:

• vratiću ti tvoju olovku = I’ll return your pencil to you

Very roughly:

• ću sounds like chyoo, but with a softer ch sound
• ti sounds like tee

So the sentence is approximately:

• PO-sle SHKO-le chyoo tee VRA-ti-ti o-LOV-ku

That is only an approximation. The Serbian ć is softer than English ch.

Yes. Serbian allows that.

For example:

• Ću ti vratiti olovku posle škole is not ideal because ću should not normally start the sentence by itself
• Ja ću ti vratiti olovku posle škole.
• Vratiću ti olovku posle škole.

These are natural.

So yes, posle škole can come later, but you still have to respect normal clitic placement.