Learn Hardcore Indonesian: Pembicara di seminar itu menunjukkan beberapa slide tentang pola belajar yang sehat. - The speaker at that seminar shows several slides about healthy study patterns.

Pembicara means “speaker” as a person, someone who is giving a talk, lecture, or presentation.
It does not mean a loudspeaker or speaker device; that would be speaker, pengeras suara, or sound system in Indonesian.
So in this sentence, pembicara di seminar itu means “the speaker at that seminar” (a person).

Why is it di seminar itu and not pada seminar itu or di dalam seminar itu? Are they all correct?

All three are grammatically possible, but there are nuances:

di seminar itu – very natural and common; literally “at that seminar.”
pada seminar itu – a bit more formal/written; also “at/on that seminar.”
di dalam seminar itu – literally “inside that seminar,” which sounds slightly odd unless you explicitly contrast with outside the seminar room.

In everyday speech, di seminar itu is the most natural choice here.

What is the difference between menunjukkan and just menunjuk? Why is menunjukkan used here?

menunjuk = “to point (at something)” with your finger, a pointer, etc.
menunjukkan = “to show / to point out / to demonstrate (something to someone).”

In this sentence, the speaker is showing slides as part of a presentation, not just physically pointing, so menunjukkan is the correct verb.
You could think of menunjukkan beberapa slide as “to present / display some slides.”

Is menunjukkan the only verb I can use for “showing slides,” or are there alternatives?

Other possible verbs include:

memperlihatkan beberapa slide – “to show some slides” (more literal “make [them] visible”).
menayangkan beberapa slide – “to display / screen some slides,” often used for visuals like videos, slides, ads.
memaparkan beberapa slide – “to present some slides” (more formal/presentation context).

Menunjukkan is neutral and very natural; the sentence as given sounds perfectly normal.

Why is it beberapa slide and not something like a plural form of slide? How is plural expressed here?

Indonesian does not change the noun form for plural.
Plurality is usually shown by:

context,
words like beberapa (some, several), banyak (many), etc.,
or sometimes reduplication (e.g. buku-buku for “books”).

Here, beberapa already indicates more than one slide, so slide stays in its base form.
There’s no need (and no correct way) to “pluralize” slide as a separate form.

Could I say beberapa buah slide? Do I need a classifier like buah for slide?

You can say beberapa buah slide, and it is grammatically correct.
However, for many modern loanwords like slide, Indonesian speakers often omit classifiers, especially in casual or semi-formal contexts.

So:

beberapa slide – very natural and common.
beberapa buah slide – slightly more explicit/careful; still correct, maybe a bit more formal or written.

What does tentang mean, and can it be placed somewhere else in the sentence?

tentang means “about / regarding / concerning.”
In this sentence: slide tentang pola belajar yang sehat = “slides about healthy study patterns.”

Its usual position is before the noun phrase it refers to, like tentang pola belajar yang sehat.
You normally wouldn’t move tentang to another part of the sentence (e.g. you don’t say “Pembicara tentang menunjukkan…”), so the word order here is standard.

Can you break down pola belajar yang sehat and explain what yang is doing here?

pola = pattern, way, style, habit.
belajar = to study / studying.
pola belajar = “study pattern” or “way of studying.”
yang sehat = “that is healthy” / “which is healthy.”

Here, yang introduces a relative clause or descriptive phrase that modifies pola belajar.
So pola belajar yang sehat literally means “a pattern of studying that is healthy”, or more naturally, “healthy study patterns.”

Does yang sehat describe the pola belajar or the pembicara? How do we know?

yang sehat is attached to pola belajar, because it comes right after that noun phrase.
The structure is:

pola belajar (noun phrase)
yang sehat (relative clause describing that noun phrase)

If it were describing pembicara, it would have to be placed after pembicara, e.g.
Pembicara yang sehat di seminar itu… – which would itself sound odd in meaning.
So grammatically and naturally, yang sehat modifies pola belajar.

Could I change the word order to Pembicara menunjukkan beberapa slide di seminar itu tentang pola belajar yang sehat? Would that still be correct?

Pembicara menunjukkan beberapa slide di seminar itu tentang pola belajar yang sehat is grammatically possible, but it sounds a bit clumsy.
Listeners might momentarily wonder if di seminar itu modifies pembicara or menunjukkan.

The original Pembicara di seminar itu menunjukkan beberapa slide tentang pola belajar yang sehat is:

smoother
clearly marks “di seminar itu” as describing pembicara (“the speaker at that seminar”), not the act of showing.

So the original order is more natural.

What’s the function of itu in seminar itu? What changes if I say just di seminar?

itu is a demonstrative meaning “that”, used here as a kind of determiner.

di seminar itu = “at that seminar” (a specific seminar both speaker and listener know about).
di seminar (without itu) = more general, like “at a seminar” or “at the seminar” in context, but less explicitly specific.

Including itu makes it clear the seminar is a particular, known seminar.

How do Indonesians know this is past tense (“showed”) and not present (“shows”)? There’s no tense marker.

Indonesian verbs do not change form for tense.
Time is understood from context, optional time words, or surrounding sentences.

In isolation, menunjukkan can be:

“shows / is showing” (present)
“showed / was showing” (past)
even “will show” (future), depending on context.

If you wanted to be explicit, you could add time words, e.g.:

Kemarin pembicara di seminar itu menunjukkan… – “Yesterday the speaker at the seminar showed…”
Nanti pembicara di seminar itu akan menunjukkan… – “Later the speaker at the seminar will show…”