Learn Hardcore Indonesian: Adik perempuan saya cemas karena nilai ujian, tetapi Ayah bilang bahwa semuanya wajar. - My younger sister is anxious because of the exam score, but Father says that everything is reasonable.

adik = younger sibling (always younger, gender-neutral)
perempuan = female
saya = my So adik perempuan saya means “my younger sister.” For an older sister, you’d say kakak perempuan saya; for a younger brother, adik laki-laki saya; for an older brother, kakak laki-laki saya.

Can I just say adik saya? And is adik saya perempuan okay?

adik saya is fine and common if the gender is clear from context or not important.
adik saya perempuan is a grammatical sentence meaning “my younger sibling is female,” but it describes a fact about your sibling; it’s not the standard way to say “my younger sister” as a noun phrase. Use adik perempuan saya for that.

Why is Ayah capitalized, and why is there no possessive like “my”?

Capitalized Ayah works like a proper name and means “Dad,” implicitly “my dad,” so you don’t need a possessive.
Lowercase ayah is a common noun “father.” To be explicit, say ayah saya (my father) or Ayahku (my dad).
You normally only capitalize kinship terms (Ayah, Ibu, Kak, Adik) when used as forms of address or as names in your family.

Is bilang informal? How does it differ from berkata and mengatakan?

bilang = “to say/tell,” colloquial and very common in speech.
berkata = “to say,” neutral/formal-ish, often used in writing or careful speech.
mengatakan = “to state/say (that),” more formal; typically takes a clause or a quoted statement. Examples:
Ayah bilang (bahwa) semuanya wajar. (informal to neutral)
Ayah berkata bahwa semuanya wajar. (neutral)
Ayah mengatakan bahwa semuanya wajar. (more formal)

Do I need bahwa after bilang? Can I use kalau?

bahwa (“that”) is optional after verbs of saying: Ayah bilang (bahwa) semuanya wajar.
In colloquial speech, kalau is often used as a complementizer: Ayah bilang kalau semuanya wajar. It’s very natural in conversation, but in formal writing prefer bahwa.

Is cemas an adjective? Why is there no word for “is” like English “to be”? Can I use adalah?

cemas is an adjective meaning “anxious.”
Indonesian adjectives can function as predicates without a copula: Adik perempuan saya cemas (“My younger sister is anxious”).
Don’t use adalah with adjectives. adalah links two nouns: Masalahnya adalah nilai ujian. (The problem is the exam scores.)

What’s the nuance difference among cemas, khawatir, gelisah, and takut?

Why karena here? How does it differ from sebab and gara-gara?

karena: “because” (neutral), used before clauses or nouns: cemas karena nilai ujian (“anxious because of exam scores”).
sebab: “because” (a bit more formal/written).
gara-gara: “because of” with a colloquial/negative flavor, often blaming a cause: cemas gara-gara nilai ujian.

What exactly does nilai ujian mean? Is it singular or plural? How about hasil ujian?

nilai ujian = exam grade/score(s). Indonesian doesn’t mark plural; context decides.
hasil ujian = exam result(s)/outcome(s), slightly broader; could include pass/fail, rankings, etc. To emphasize plural, you can say semua nilai ujian (all the exam scores).

How do I make it explicit that the scores are her scores?

Use a possessor:

Is the comma before tetapi required? Could I use tapi or namun instead?

A comma before tetapi is standard because it links two independent clauses.
tapi is the common colloquial form; also takes a comma: ..., tapi ...
namun is more formal and usually starts the next clause/sentence: Namun, Ayah bilang ...

Can I move the karena phrase to the front?

Yes:

Karena nilai ujian, adik perempuan saya cemas, tetapi Ayah bilang bahwa semuanya wajar. Fronting the reason is natural in Indonesian; just keep the comma.

What does semuanya refer to here? How is it different from semua? Any alternatives?

semuanya is a pronoun meaning “everything/all of it,” referring to the whole situation.
semua is a determiner used before nouns: semua hal (all things). Alternatives:
Itu hal yang wajar. (That’s a normal thing.)
Wajar saja. / Itu wajar kok. (That’s perfectly normal — the latter is casual.)
Semuanya baik-baik saja. (Everything is fine.)

Could you show a more casual version and a more formal version of the whole sentence?

Casual: Adik aku cemas gara-gara nilai ujian, tapi Ayah bilang semuanya wajar.
Neutral (original is already neutral): Adik perempuan saya cemas karena nilai ujian, tetapi Ayah bilang bahwa semuanya wajar.
More formal: Adik perempuan saya cemas karena nilai ujian, namun Ayah mengatakan bahwa semuanya wajar.