Learn Hardcore Indonesian: Teman saya mengikuti les biola setiap Sabtu pagi. - My friend attends violin lessons every Saturday morning.

temanku = my friend (colloquial, using the suffix -ku)
teman aku = also possible in casual speech, but teman saya is more neutral/standard.

Does teman saya clearly mean one friend, or could it also mean my friends (plural)?

Indonesian usually does not mark singular vs plural on nouns. So:

teman saya can mean my friend or my friends, depending on context.

If you really want to be explicit:

seorang teman saya = one of my friends / a (single) friend of mine
teman-teman saya = my friends (clearly plural, by reduplicating teman)

What exactly does mengikuti mean here? Why not just ikut?

The root verb is ikut, which means to follow, to join, to participate.

mengikuti is the me- form of ikut, often meaning to follow (something), to attend, to take part in.

In this sentence:

mengikuti les biola ≈ to attend violin lessons / to take violin lessons.

You could say:

Teman saya ikut les biola setiap Sabtu pagi.

This is also natural and maybe a bit more casual. Mengikuti sounds a bit more formal or “complete,” but both are correct.

What is les exactly? How is it different from words like pelajaran or kursus?

Les is a loanword (from Dutch les, and similar to English lessons). It usually means:

private lessons, tutoring, or extra classes outside regular school.

Typical uses:

les biola = violin lessons
les piano, les bahasa Inggris, les matematika, etc.

Differences:

pelajaran = a lesson/subject in a more general or school context (e.g., pelajaran matematika = math class/subject).
kursus = a course, often more structured, sometimes longer-term or at an institution (e.g., kursus bahasa Jepang = a Japanese course).

Here, les biola suggests private or extra violin lessons, not just a regular school subject.

Why is there no word for takes or has (as in “takes violin lessons”)?

In Indonesian, mengikuti (from ikut) already covers the meaning of taking/attending a class. You don’t need an extra word like has or takes.

mengikuti les biola = to attend/take violin lessons

Indonesian often uses one verb where English would use a “light verb” like have/take/give + noun.

Why is there no word for a or the in les biola?

Indonesian doesn’t use articles like a/an or the. Nouns stand alone:

les biola can mean a violin lesson, the violin lesson, or violin lessons, depending on context.

If you need to be more specific, you use other words:

sebuah les biola = a violin class (this sounds a bit unusual; sebuah is rarely used with les)
les biola itu = that/the violin lesson (using itu as a demonstrative)

Why is it setiap Sabtu pagi and not setiap pagi Sabtu?

The natural order for this kind of expression is:

setiap + [day] + [time of day]

So:

setiap Sabtu pagi = every Saturday morning

Other natural patterns:

setiap Sabtu = every Saturday
setiap pagi = every morning
setiap hari Sabtu pagi = every Saturday morning (a bit longer, but also correct)

setiap pagi Sabtu is not how Indonesians normally say it and sounds off.

Why is Sabtu capitalized? Are days of the week always capitalized in Indonesian?

Yes. In standard Indonesian spelling:

Names of days and months are capitalized.

So you write:

Senin, Selasa, Rabu, Kamis, Jumat, Sabtu, Minggu

You’ll sometimes see lowercase in informal writing (social media, texts), but the correct, formal rule is to capitalize them.

Why is there no di before Sabtu pagi? In English we say on Saturday morning.

In Indonesian, for time expressions like days, dates, and parts of the day, you often don’t use a preposition at all:

Sabtu pagi = Saturday morning
Senin sore = Monday afternoon
jam lima pagi = 5 in the morning

You can sometimes use pada with time expressions, especially in more formal Indonesian:

pada Sabtu pagi = on Saturday morning

But in everyday speech, Sabtu pagi without di or pada is perfectly natural and very common. Using di Sabtu pagi is not idiomatic.

What is the nuance of pagi here? Does Sabtu pagi mean on Saturday morning or every morning on Saturday?

pagi means morning.

Sabtu pagi = Saturday morning (one specific morning)
In the sentence with setiap: setiap Sabtu pagi = every Saturday morning.

It doesn’t mean “every morning on Saturday” (which is strange in English anyway). It means: on each Saturday, in the morning.

Does mengikuti show present tense here? How are tenses handled in this sentence?

Indonesian verbs do not change form for tense. Mengikuti has the same form whether you mean:

follows / is following / followed / will follow

Tense or time is usually understood from:

context
time words, like kemarin (yesterday), besok (tomorrow), setiap (every), etc.

In this sentence, setiap Sabtu pagi (every Saturday morning) tells us it’s a habitual action, so the natural English translation is:

My friend takes (or attends) violin lessons every Saturday morning.

Could we say Teman saya belajar biola setiap Sabtu pagi instead? What’s the difference?

Yes, you can say:

Teman saya belajar biola setiap Sabtu pagi.
= My friend studies/learns violin every Saturday morning.

Differences in nuance:

mengikuti les biola emphasizes attending a structured lesson/class with a teacher.
belajar biola is more general: learning/studying violin (could be self-study, could be with a teacher, not specified).

Both are grammatical; your choice depends on what you want to emphasize.

How formal or informal is this sentence? Would it sound natural in casual conversation?

The sentence:

Teman saya mengikuti les biola setiap Sabtu pagi.

is neutral to slightly formal mainly because of mengikuti and saya.

Casual variants:

Teman aku ikut les biola tiap Sabtu pagi.
Temanku ikut les biola tiap Sabtu pagi.

Changes:

aku or -ku instead of saya ⇒ more informal
tiap instead of setiap ⇒ more conversational
ikut instead of mengikuti ⇒ more relaxed, everyday speech

All versions are natural; choice depends on context and who you’re talking to.