Learn Hardcore Serbian: Sutra će moja sestra posle škole učiti jezik na internetu. - Tomorrow my sister will study a language on the internet after school.

Breakdown of Sutra će moja sestra posle škole učiti jezik na internetu.

What tense is će učiti, and how is it formed?

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

In this sentence, it is formed with:

So će učiti means will learn / will study.

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

Why is će placed right after Sutra?

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.

Can this future tense also be written as one word, like učiće?

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

So you may see both:

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