Learn Hardcore Indonesian: Teman saya belajar drum di studio musik. - My friend learns the drums at the music studio.

Teman saya belajar drum di studio musik.
can be understood as:
- My friend is learning drums at a music studio.
  or
- A friend of mine is learning drums at a music studio.

If you really want to emphasize one (single) friend of mine, you can say:

Seorang teman saya belajar drum di studio musik.
(One friend of mine studies/learns drums at a music studio.)

Is teman singular or plural here? How do I say my friends?

By itself, teman is neutral: it can be friend or friends, depending on context.

In this sentence:

Teman saya belajar drum di studio musik.

it most naturally sounds like my friend (one person), but it could be my friends if the context made that clear.

To be more explicit, you can say:

Seorang teman saya = a friend of mine (clearly singular)
Teman-teman saya = my friends (clearly plural; repetition marks plural)
Banyak teman saya = many of my friends

Is teman gendered like friend vs girlfriend/boyfriend? How do I know if it’s male or female?

Teman is completely gender-neutral. It just means friend.

If you want to specify gender, you add extra words:

teman laki-laki = male friend
teman perempuan or teman cewek = female friend (the second is informal/colloquial)

But in daily speech, people often leave it as teman and let context explain who it is.

Why isn’t there any word for is (like in My friend is learning)? Is something missing?

Nothing is missing. Indonesian normally does not use a separate verb like to be (is/are/am) before another verb.

The verb belajar already carries the meaning to learn / to study, so you just say:

Teman saya belajar drum.
literally: My friend learn(s) drums.

There is no extra is needed. You only use forms like adalah in different structures, mainly to link a subject to a noun phrase, not to an action verb:

Dia adalah guru. = He/She is a teacher.
Dia belajar. = He/She studies / is studying. (no adalah here)

How do we know if this sentence is present, past, or future? Could it mean was learning or will learn?

Indonesian verbs do not change form for tense. Belajar can cover past, present, or future. The exact time is understood from context or from time words.

Your sentence could translate as:

My friend learns drums at a music studio.
My friend is learning drums at a music studio.
My friend learned drums at a music studio.
My friend will learn drums at a music studio.

To be clearer, you can add time markers:

Kemarin teman saya belajar drum di studio musik.
Yesterday my friend learned drums at a music studio.
Sekarang teman saya belajar drum di studio musik.
Now my friend is learning drums at a music studio.
Besok teman saya akan belajar drum di studio musik.
Tomorrow my friend will learn drums at a music studio.

How would I say is currently learning or is in the middle of learning?

You can add an aspect marker like sedang or lagi to show a currently in progress action.

Teman saya sedang belajar drum di studio musik.
Teman saya lagi belajar drum di studio musik. (more informal)

Both mean roughly: My friend is currently learning drums at a music studio.

Without sedang or lagi, it can still be present tense, but it’s more neutral and can also express a general habit.

Why is it belajar drum? Shouldn’t it be belajar bermain drum for learn to play the drums?

Both are correct; the shorter version is very natural:

Teman saya belajar drum di studio musik.
Teman saya belajar bermain drum di studio musik.

In everyday speech, Indonesians often use:

belajar + [skill noun]
belajar gitar, belajar piano, belajar bahasa Indonesia, belajar komputer

Adding bermain (to play) makes it a bit more explicit:

belajar bermain drum = learn to play the drums
belajar bermain gitar = learn to play the guitar

But it’s not required; belajar drum already clearly suggests learning to play the drums.

Could I use another verb like mempelajari instead of belajar here?

Not naturally, no.

Belajar is used for learning or studying in general, and it can take nouns like school subjects, skills, instruments, etc.
Mempelajari is more formal/academic and often means to study something in depth (as a subject of research).

For example:

Dia mempelajari biologi laut.
He/She studies marine biology (as a field of study).

For learning a practical skill like playing an instrument, you normally use:

belajar drum, belajar gitar, belajar menyetir (learn to drive)

So Teman saya belajar drum di studio musik. is the natural choice.

Why do we use di before studio musik? What’s the difference between di and ke?

Di marks location (at / in / on), while ke marks movement (to).

In your sentence, the friend is located at the music studio while learning, so we use di:

di studio musik = at a music studio

Compare:

Teman saya belajar drum di studio musik.
My friend learns drums at a music studio.
Teman saya pergi ke studio musik.
My friend goes to the music studio.
Teman saya pulang dari studio musik.
My friend goes home from the music studio. (dari = from)

Why is it studio musik and not musik studio? How does noun order work here?

In Indonesian noun phrases, the main noun usually comes first, and the modifier comes after it.

So you get:

studio musik = studio (of) music → music studio
guru musik = music teacher
toko buku = book shop
rumah sakit = sick house → hospital

Saying musik studio would sound wrong; it would be like saying music of studio without a clear head noun-first structure.

So studio musik is the correct and natural order.

Why is there no a or the in studio musik? How do you say a music studio vs the music studio?

Indonesian does not have normal articles like a/an or the. The bare noun phrase studio musik can be translated as:

a music studio
the music studio

The exact meaning comes from context.

If you want to emphasize one music studio, you can optionally add a classifier word:

sebuah studio musik = a music studio (one studio; sebuah is a general classifier)

But in normal conversation, people often just say studio musik and let context decide whether it’s a or the.