Learn Hardcore Indonesian: Salah satu teman saya belajar di perpustakaan malam ini. - One of my friends is studying at the library tonight.

Satu teman saya literally means one friend of mine, and sounds a bit odd or incomplete in Indonesian when you want to say one of my friends (from a group).

Salah satu teman saya is the natural way to say one of my friends.

salah satu = one of (several)
So Salah satu teman saya implies you have multiple friends, and you are talking about one member of that group.

But doesn’t salah mean wrong or mistake? Why is it used in salah satu?

Yes, on its own salah usually means wrong / incorrect.

However, in the fixed phrase salah satu, it no longer has that meaning.

salah satu is an idiomatic expression meaning one of (them).

Native speakers don’t feel the meaning “wrong” here; they just understand salah satu as a unit meaning one of.

Is teman saya singular or plural? Does it mean my friend or my friends?

By itself, teman saya is ambiguous: it can mean either my friend or my friends, depending on context. Indonesian doesn’t mark plural with an s like English.

In salah satu teman saya:

We know teman saya is conceptually plural (my friends)
Because salah satu means one of (several)

So the whole phrase means one of my friends (implying that I have more than one friend).

Could I say salah satu dari teman saya or salah satu dari teman-teman saya instead? Are they different?

Yes, you can, and they’re all grammatically correct, with slight nuances:

salah satu teman saya
- Very common and natural.
- Slightly more compact.
salah satu dari teman saya
- Literally: one from my friends.
- Also correct, just a bit longer; feels slightly more explicit or formal.
salah satu dari teman-teman saya
- teman-teman = plural friends.
- Emphasises plurality: one of my (many) friends.

All three are fine; salah satu teman saya is the most typical everyday version.

How do I know if belajar here means is studying or will study? There’s no tense.

Indonesian verbs don’t change form for tense, so belajar can mean study / is studying / will study / studied, depending on context.

In Salah satu teman saya belajar di perpustakaan malam ini:

malam ini = tonight / this evening
This usually refers to a time slightly in the future (later this evening),
so the natural English translation is:
- One of my friends will study at the library tonight, or
- One of my friends is studying at the library tonight (if you’re talking about a plan).

Context decides whether the nuance is more present-progressive or future.

Should I add sedang to show is studying right now?

You can add sedang if you want to emphasise that the studying is in progress at that moment.

Salah satu teman saya sedang belajar di perpustakaan.
- One of my friends is studying at the library (right now).

In your original sentence:

malam ini usually points to later today / tonight, not necessarily right now.
So if the friend is studying at this very moment, you might say:
Salah satu teman saya sekarang sedang belajar di perpustakaan.
(sekarang = now)

But without sedang, the sentence is still natural; Indonesian often leaves it out.

Can I move malam ini to another position in the sentence?

Yes. Common options:

Salah satu teman saya belajar di perpustakaan malam ini.
- Neutral / very natural.
Malam ini salah satu teman saya belajar di perpustakaan.
- Emphasises tonight (as the topic): “As for tonight, one of my friends…”
Salah satu teman saya malam ini belajar di perpustakaan.
- Possible, but less common in everyday speech; sounds a bit more marked or literary.

Most of the time, learners are safest with the original:

… belajar di perpustakaan malam ini.

Why is it di perpustakaan, not ke perpustakaan?

di and ke are different:

di = at / in / on (location, where something is)
ke = to (direction, movement toward a place)

In your sentence:

belajar di perpustakaan = study at the library (location)
If you talked about going there, you’d use ke:
Teman saya pergi ke perpustakaan.
My friend went to the library.

Indonesian doesn’t have articles like a/the. How do we know if perpustakaan means the library or a library?

You’re right: Indonesian has no a / an / the.

Perpustakaan simply means library. Whether you translate it as a library or the library in English depends on context:

If both speakers know which library is meant, English will usually use the library.
If it’s just any library, you might say a library.

Indonesians rely on shared context instead of articles. If you really need to be specific, you can add words like itu (that) or a name:

di perpustakaan itu = at that library
di Perpustakaan Kota = at the City Library

Could I drop saya and just say Salah satu teman belajar di perpustakaan malam ini?

You can say it, but the meaning changes:

Salah satu teman saya…
- One of my friends…
Salah satu teman…
- One of the friends… (not clear whose friends—could be any group previously mentioned)

So if you specifically mean my friends, you should keep saya:

Salah satu teman saya belajar di perpustakaan malam ini.

What’s the difference between malam ini and nanti malam?

Both relate to tonight, but with a nuance:

malam ini
- Literally this night / this evening
- Neutral tonight, often with a sense of “today’s evening” as a whole.
nanti malam
- Literally later tonight
- Emphasises later (from now) on the same night.

In most everyday contexts, you can often use either, but:

Talking about a schedule: Saya ada janji malam ini. (I have an appointment tonight.)
Talking about something later: Kita makan di luar nanti malam. (We’ll eat out later tonight.)

Can I say salah seorang teman saya instead of salah satu teman saya?

Yes, and it adds a bit of precision and a slight formality:

seorang is a classifier for people (one person).
salah seorang teman saya literally: one person among my friends.

Both are correct:

Salah satu teman saya belajar di perpustakaan malam ini.
Salah seorang teman saya belajar di perpustakaan malam ini.

Salah seorang can feel a bit more formal or careful; salah satu is very common and neutral.

Why don’t we use yang here, like salah satu teman saya yang belajar di perpustakaan malam ini?

Adding yang changes the structure and meaning.

Your sentence:

Salah satu teman saya belajar di perpustakaan malam ini.
= One of my friends will study / is studying at the library tonight.
(belajar is the main verb of the sentence.)

With yang:

Salah satu teman saya yang belajar di perpustakaan malam ini…
= One of my friends who studies / is studying at the library tonight…
This is now a noun phrase with a relative clause, and it sounds incomplete unless you finish it:
- Salah satu teman saya yang belajar di perpustakaan malam ini akan pulang terlambat.
  One of my friends who is studying at the library tonight will go home late.

So in your original sentence, yang is not needed because belajar is the main verb, not part of a relative clause.