Obuka počinje sutra.

Obuka is in the nominative singular because it is the subject of the sentence, the thing that is doing the action of starting.

It is also a feminine noun. A very common clue is the ending -a, which often marks feminine nouns in Serbian.

Because the subject obuka is singular. Serbian verbs agree with the subject in person and number.

So:

• obuka = one training course/session
• therefore počinje = it starts

Serbian does not need a separate word for it here.

In Serbian, the present tense is often used for a planned, scheduled, or expected future event, especially when a time word like sutra makes the future meaning clear.

So Obuka počinje sutra is perfectly natural, just like English The training starts tomorrow.

The form počinje comes from the verb počinjati, which is the imperfective verb meaning to be starting / to start in an ongoing or general sense.

Its perfective partner is početi.

This is a useful pair to know:

• počinjati = imperfective
• početi = perfective

In this sentence, the imperfective present sounds natural because it describes a scheduled event.

Yes. Obuka će početi sutra is also correct and very natural.

The difference is mostly one of style and nuance:

• Obuka počinje sutra = present tense used for a scheduled future event
• Obuka će početi sutra = explicit future tense

Both mean the same thing in practice here.

Because sutra already means tomorrow by itself. It functions as a time adverb, so no preposition is needed.

This is similar to English:

• tomorrow
• not on tomorrow

So Serbian simply says sutra, not something like na sutra.

Yes. Serbian word order is fairly flexible.

The most neutral version here is:

• Obuka počinje sutra.

You could also say:

• Sutra počinje obuka.

That version puts more focus on tomorrow.

The meaning stays basically the same, but the emphasis can shift depending on word order.

A rough English-friendly guide is po-CHIN-ye.

Two important sound points:

• č sounds like ch in church
• nj sounds like ny in canyon

So počinje has that ny sound in the last part.

Because Serbian has no articles like a, an, or the.

So obuka can mean:

• training
• the training
• sometimes a training course/session

The exact meaning depends on context.

Yes, if the context already makes it clear what is starting.

Serbian often leaves out things that are already understood. So if everyone knows you are talking about the training, Počinje sutra is natural and means It starts tomorrow.

But if you are introducing the information for the first time, Obuka počinje sutra is clearer.