Learn Hardcore Indonesian: Pelatih olahraga yang saya kenal juga bermain drum di band kampus pada akhir pekan. - The sports coach whom I know also plays drums in the campus band on weekends.

Pelatih olahraga = the sports coach
saya kenal = I know
yang links them: pelatih olahraga yang saya kenal = the sports coach (whom) I know

So the structure is:

[noun] + yang + [clause describing that noun]

In English we put who/that before the verb: the coach who I know.
In Indonesian you always keep yang and then a full clause: yang saya kenal (literally: that I know).

Is saya the subject of the main sentence, or is pelatih olahraga the subject?

The subject of the main sentence is pelatih olahraga yang saya kenal (the whole noun phrase).

Breakdown:

Pelatih olahraga yang saya kenal = subject
juga bermain drum di band kampus pada akhir pekan = predicate

Inside the relative clause yang saya kenal, saya is the subject of kenal:

inner clause: saya kenal (pelatih olahraga) = I know (the coach)
main clause: [that coach] also plays drums …

So:

Main subject: pelatih olahraga yang saya kenal
Inner-clause subject: saya (inside yang saya kenal)

Why is the order pelatih olahraga yang saya kenal and not something like yang saya kenal pelatih olahraga?

In Indonesian, the relative clause always comes after the noun it describes:

pattern: [NOUN] + yang + [CLAUSE]

So the correct order is:

pelatih olahraga (the noun being described)
yang
saya kenal (the clause that describes that noun)

Putting yang before the noun as in yang saya kenal pelatih olahraga is ungrammatical.
Relative clauses in Indonesian never go in front of the noun the way they sometimes can in English (e.g. The one I know, the coach…). You must keep:

pelatih olahraga yang saya kenal

What exactly does pelatih olahraga mean? Is olahraga like an adjective here?

Olahraga literally means sport, and here it functions as a noun modifier (like a compound noun), not an adjective in the strict grammatical sense.

pelatih = coach / trainer
olahraga = sport(s)
pelatih olahraga = sports coach / athletic coach

This noun + noun pattern is very common in Indonesian and often corresponds to English compounds:

guru bahasa = language teacher
pelatih renang = swimming coach
tiket pesawat = plane ticket

So olahraga works similarly to sports in sports coach.

What does juga do here, and why is it placed before bermain?

Juga means also / too / as well.

In this sentence:

juga bermain drum = also plays the drums

The neutral and most common position is before the main verb or verb phrase:

Dia juga bermain drum. = He also plays drums.

You can sometimes move juga a bit, but:

pelatih olahraga yang saya kenal juga bermain drum ...
sounds more natural than
pelatih olahraga yang saya kenal bermain drum juga ... (this can sound like he plays drums too (besides something else), with a slightly different focus)

So juga here adds the idea that in addition to being a sports coach, he also plays drums.

Can I say main drum instead of bermain drum?

Yes, main drum is very common in everyday speech.

bermain drum = more complete / slightly more formal or careful
main drum = colloquial, very widely used

Both mean to play the drums.

You might also see memainkan drum, but that sounds more like to play (an instrument) as an object and is less common in casual speech for this context. For talking about what someone does (their activity/hobby), bermain drum or main drum is more natural.

Why is it di band kampus? Could it be just band kampus without di?

Di is a preposition meaning in / at / on (location).

bermain drum di band kampus
literally: play drums in the campus band

Here:

band kampus = campus band (a band belonging to / associated with a campus)
di band kampus = in a campus band

If you drop di, you get bermain drum band kampus, which is ungrammatical; the verb bermain drum needs a preposition before a place or group you play in/at.

Compare:

Dia bekerja di bank. = He works at a bank.
Dia bermain di band kampus. = He plays in the campus band.

What does band kampus mean exactly? Is kampus like an adjective here?

Kampus is a noun meaning campus, used as a modifier after band:

band = band (music group)
kampus = campus
band kampus = campus band (a band from/at the campus)

This is the same noun + noun pattern as with pelatih olahraga. Indonesian usually puts the main noun first, then the noun that describes or limits it:

band kampus = campus band
mobil kantor = office car (company car)
perpustakaan kota = city library

So kampus is still a noun, functioning like an English noun modifier.

What is the difference between pada akhir pekan and di akhir pekan?

Both are used and understood, but there is a nuance:

pada akhir pekan
- more formal / standard in writing
- pada often used for time expressions
di akhir pekan
- very common in everyday speech
- di is now widely used for both place and time in modern Indonesian, although traditionally di is for place and pada for time.

In conversation, di akhir pekan is extremely common. In careful written Indonesian (e.g. essays, news), pada akhir pekan is often preferred.

You can also just say:

akhir pekan
e.g. Dia bermain drum di band kampus akhir pekan.
(casual, with the time phrase left bare at the end)

Where can I put pada akhir pekan in the sentence? Must it be at the end?

It doesn’t have to be at the end. Indonesian is flexible with time expressions. All of these are possible:

Pelatih olahraga yang saya kenal juga bermain drum di band kampus pada akhir pekan.
Pada akhir pekan, pelatih olahraga yang saya kenal juga bermain drum di band kampus.
Pelatih olahraga yang saya kenal pada akhir pekan juga bermain drum di band kampus. (less common, because it can sound like I know him on weekends, not always)

Most natural are:

Time at the beginning: Pada akhir pekan, …
Time at the end: … di band kampus pada akhir pekan.

Putting it in the middle can change the focus or create slight ambiguity.

Is pada akhir pekan more like on the weekend or on weekends?

Pada akhir pekan by itself is neutral; context decides whether it’s this weekend / that weekend or weekends in general.

In your sentence:

Pelatih olahraga yang saya kenal juga bermain drum di band kampus pada akhir pekan.

The natural interpretation is on weekends / at the weekend (habitually), because we’re talking about a regular activity (playing in a campus band).

If you want to emphasize every weekend, you can say:

pada setiap akhir pekan = every weekend
setiap akhir pekan = every weekend

Could I say pelatih olahraga yang kukenal instead of yang saya kenal?

Yes, you can:

yang saya kenal = that I know
yang kukenal = that I know (with aku combined into ku-)

Details:

ku- is the bound form of aku (I), used as a prefix attached to verbs:
- aku kenal → kukenal
yang kukenal is colloquial / informal, and sounds closer / more personal.
yang saya kenal is neutral and works in both informal and formal contexts.

Both are correct; choice depends on formality and personal style.

Can I drop saya and just say pelatih olahraga yang kenal?

No, pelatih olahraga yang kenal is ungrammatical because kenal needs a subject.

You need someone who does the knowing:

yang saya kenal = that I know
yang dia kenal = that he/she knows
yang mereka kenal = that they know
yang kukenal = that I know (informal, using ku-)

Without a subject (saya / dia / ku- / etc.), yang kenal is incomplete. So you must keep saya (or another subject form).

How would I say the sports coach that knows me instead of that I know?

You change the subject inside the relative clause:

yang saya kenal = that I know
yang kenal saya = that knows me

So:

Pelatih olahraga yang saya kenal
= the sports coach that I know
Pelatih olahraga yang kenal saya
= the sports coach that knows me

In yang kenal saya, the implied subject of kenal is pelatih olahraga (the coach), and saya is the object.

Is pelatih olahraga yang saya kenal definite (like the coach I know) or indefinite (like a coach I know)?

Indonesian doesn’t mark definiteness (the/a) the way English does. Pelatih olahraga yang saya kenal can correspond to either:

the sports coach that I know
or
a sports coach that I know

Context decides:

If you and the listener both know which coach you’re talking about, it will feel like the coach.
If you’re introducing him for the first time, it can feel like a coach I know.

If you want to emphasize definiteness, you can add itu:

Pelatih olahraga yang saya kenal itu juga bermain drum…
= That / The sports coach I know also plays drums… (very clearly a specific one)

Is the sentence overall formal, informal, or neutral?

It’s neutral, leaning slightly to standard:

Uses saya (neutral/formal I) instead of aku / gue (informal).
Uses the complete form bermain drum instead of just main drum.
Uses pada akhir pekan, which is somewhat more formal than di akhir pekan in careful writing.

In everyday spoken Indonesian, people might say:

Pelatih olahraga yang aku kenal juga main drum di band kampus di akhir pekan.

Your original sentence is perfectly natural in writing and polite speech.