Sutra će moja sestra posle škole učiti jezik na internetu.

It is the future tense in Serbian, usually called futur I.

In this sentence, it is formed with:

• the short future auxiliary će
• plus the infinitive učiti

So će učiti means will learn / will study.

Here, će is the 3rd person singular form, because the subject is moja sestra.

Because će is a clitic: a short, unstressed word that usually goes in second position in the sentence or clause.

In Sutra će moja sestra..., the first element is Sutra, so će comes right after it.

This is very normal in Serbian. You could also say:

Moja sestra će sutra posle škole učiti jezik na internetu.

That is also correct; it just puts the focus a little differently.

Yes. Serbian can also use a fused future form such as učiće.

So you may see both:

• će učiti
• učiće

Both express the future. In a sentence like yours, the separate form će učiti is very common and easy for learners to recognize.

Because moja must agree with sestra in gender, number, and case.

• sestra is feminine singular
• so the possessive adjective must also be feminine singular
• that gives moja sestra

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

Because the preposition posle requires the genitive case.

The noun škola changes like this:

• nominative: škola
• genitive singular: škole

So:

• posle škole = after school

This is a very common pattern in Serbian:

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

Because jezik is the direct object of učiti, so it is in the accusative case.

However, jezik is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: jezik
• accusative: jezik

That is why there is no visible change.

Here internetu is in the locative case.

The preposition na can take different cases, but when it means on / at in a static sense, it usually takes the locative.

So:

• na internetu = on the internet / online

The noun changes like this:

• nominative: internet
• locative: internetu

In this sentence, učiti means learn or study.

With a subject like moja sestra and an object like jezik, the meaning is clearly that she is studying or learning a language.

This verb can sometimes also mean teach, depending on the structure, so context matters. But in učiti jezik, learners should usually understand it as study/learn a language.

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

So nouns appear without articles:

• sestra
• škola
• jezik
• internet

Whether something is a language or the language, a sister or the sister, is understood from context, or made clearer with other words if needed.

No. Serbian word order is fairly flexible.

This sentence is natural, but other versions are also possible, for example:

Moja sestra će sutra posle škole učiti jezik na internetu.

The basic meaning stays the same, but the focus or emphasis can change.

The main thing to remember is that clitics like će usually have to stay in their usual second-position slot.