Learn Hardcore Indonesian: Kecuali saat saya sibuk, saya membaca buku setiap malam. - Unless I am busy, I read a book every night.

Yes. Kecuali saat saya sibuk literally means “except when I’m busy,” and in this habitual statement it’s equivalent to unless I’m busy. Variants:

kecuali saat/ketika saya sibuk = except when I’m busy (time-based)
kecuali kalau/jika/bila saya sibuk = unless I’m busy (condition-based)

Avoid bare kecuali saya sibuk; attach it to saat/ketika or kalau/jika.

Why use saat here? Could I use ketika or waktu instead?

All three can mean “when (time)”:

saat: slightly formal/neutral
ketika: neutral, common in written/narrative styles
waktu: everyday/informal Very casual is pas. All work here: Kecuali ketika/waktu saya sibuk...

Can I say pada saat or di saat instead of just saat?

Yes.

saat is the simplest and perfectly fine.
pada saat is more formal/emphatic (“at the time when”).
di saat is common in speech; some style guides prefer pada saat.

Do I need sedang or lagi with sibuk?

Not required. Saat saya sibuk already marks the time. Use them only to stress “in the middle of being busy”:

Neutral/formal: saat saya sedang sibuk
Informal: kecuali pas lagi sibuk

Is the comma necessary after the initial clause?

Yes. When an adverbial/exception clause comes first, Indonesian convention uses a comma: Kecuali saat saya sibuk, ...
If you move the exception to the end, a comma is optional: Saya membaca buku setiap malam(,) kecuali saat saya sibuk.

Can I move the exception to the end?

Yes, both orders are natural:

Kecuali saat saya sibuk, saya membaca buku setiap malam.
Saya membaca buku setiap malam, kecuali saat saya sibuk. Fronting slightly highlights the exception.

Why membaca and not baca?

Root: baca.
Active transitive: meN- + baca → membaca (meN- becomes mem- before b).
In casual speech you can use the bare root: Saya baca buku... is very common informally.

What tense is membaca? Does it mean present?

Indonesian has no verb tense. Saya membaca can be past/present/future; here setiap malam makes it habitual. For an ongoing action, add sedang: Saya sedang membaca.

Do I need an article like “a” (e.g., sebuah buku)?

No articles in Indonesian. buku is generic/unspecified. To be specific:

One book: sebuah buku or satu buku
Some books: beberapa buku
Plural/general: buku-buku or just buku (context usually suffices)

Can I omit buku and just say “I read every night”?

Yes: Saya membaca setiap malam is fine. membaca can be used without an explicit object when the activity is understood.

Can I drop saya in the first clause?

Yes, in the exception clause it’s fine: Kecuali saat sibuk, saya membaca... (the subject “I” is understood).
Don’t drop it from the main clause: ... membaca buku setiap malam sounds incomplete.

What’s the difference between setiap and tiap?

They both mean “every.” setiap is neutral/standard; tiap is a bit more colloquial/shorter. Saya baca buku tiap malam is very natural in speech.

Is malam the same as “evening”? What about malam hari?

malam = night. setiap malam is the most natural here.
malam hari = “nighttime,” more formal/explicit.
For “evening,” use sore or petang: setiap sore/petang.

Can I use selain instead of kecuali?

Sometimes. Selain can mean “apart from/other than,” and can function like “except” (e.g., Selain hari Minggu, kantor buka). But it can also mean “besides/in addition to,” which risks ambiguity. For a clear exception, kecuali is safer: ... kecuali saat saya sibuk.

Is there a register change if I use aku instead of saya?

Yes. saya is neutral/formal; aku is informal/intimate. A colloquial version is: Kecuali pas lagi sibuk, aku baca buku tiap malam.