Ne znam tačno kada počinje predstava.

Because Serbian usually drops subject pronouns when the verb already shows who the subject is.

• znam = I know
• the ending -m already tells you it is I

So Ne znam tačno kada počinje predstava. is completely natural.
You could say Ja ne znam tačno kada počinje predstava if you want extra emphasis on I.

Here is a word-by-word breakdown:

• Ne = not
• znam = I know
• tačno = exactly, precisely
• kada = when
• počinje = starts, is starting, begins
• predstava = show, performance, play

So the structure is literally:

• Ne znam = I do not know
• tačno kada = exactly when
• počinje predstava = the show starts

Because Serbian uses znati for knowing a fact or piece of information, and poznavati / poznajem for being familiar with a person, place, or thing.

In this sentence, you are talking about knowing when something happens, so you need znati:

• Ne znam kada počinje predstava. = I don’t know when the show starts.

But:

• Poznajem tog glumca. = I know that actor.
• Ne poznajem Beograd dobro. = I don’t know Belgrade well.

So znam is the correct verb for information.

In Serbian, just like in English, the present tense is often used for scheduled future events.

So predstava počinje u osam means:

• the show starts at eight
• or the show is starting at eight

This is normal for timetables, programs, arrivals, departures, and similar planned events.

That is why kada počinje predstava sounds natural, even if the performance has not started yet.

Predstava is the subject of the clause kada počinje predstava, so it stays in the nominative case.

Basic form:

• predstava = performance / show / play

Because it is the thing that is doing the action of starting, no case change is needed here.

Compare:

• Predstava počinje. = The show starts.
• Gledam predstavu. = I am watching the show.

In the second sentence, predstavu is the object, so it changes case.

Serbian has no articles, so there is no separate word for a or the.

That means predstava can mean:

• a show
• the show

The exact meaning depends on context.

In this sentence, English naturally uses the show, but Serbian simply says predstava.

Here tačno means exactly or precisely.

So:

• Ne znam tačno kada počinje predstava.
  means
• I don’t know exactly when the show starts.

Yes, the position can vary a little. For example:

• Ne znam kada tačno počinje predstava.

This also sounds natural and means essentially the same thing.
The difference is mostly about emphasis and rhythm, not basic meaning.

Yes. Kad is a shorter, very common variant of kada.

So you can say:

• Ne znam tačno kada počinje predstava.
• Ne znam tačno kad počinje predstava.

Both are correct.
Very generally:

• kada can sound a bit more neutral or slightly more formal
• kad is very common in everyday speech

Yes, Serbian word order is more flexible than English word order.

The given sentence is very natural:

• Ne znam tačno kada počinje predstava.

But you may also hear:

• Ne znam kada tačno počinje predstava.

Both are fine.

Serbian often moves words around to adjust emphasis, focus, or style.
However, the original order is a very standard, neutral way to say it.

A rough pronunciation guide:

• počinje ≈ PO-chee-nye
• tačno ≈ TACH-no

A few helpful notes:

• č sounds roughly like ch in chocolate
• nj is like the ny sound in canyon or Spanish ñ
• tačno has čn, so the ch sound is followed right away by n

These are only approximations, but they are good starting points for an English speaker.

Počinje is the form used here because it sounds natural for a regular, factual statement about when something starts, especially a scheduled event.

• kada počinje predstava = when the show starts / when the show is starting

Počne comes from a different aspect and is often used in slightly different contexts, often focusing more on the single moment of beginning.

For a learner, the safest thing is:

• use počinje for neutral statements like schedules and start times
• learn počne later as part of aspect contrasts

So in this sentence, počinje is exactly what you would expect.

It is neutral and works well in both everyday conversation and normal standard language.

• not slang
• not especially formal
• completely natural in speech and writing

If you wanted it to sound a little more casual, you might use kad instead of kada:

• Ne znam tačno kad počinje predstava.

But the original sentence is already very natural.