Learn Hardcore Indonesian: Namun keponakan perempuan saya tidak marah; dia takut badai saja. - However my female niece is not angry; she is just afraid of the storm.

Tetapi = but (neutral–formal)
Tapi = but (informal) All three can start a sentence: e.g., Namun, ... / Tetapi, ... or in the middle: Saya mau pergi, namun hujan.

Should there be a comma after Namun?

By Indonesian style guidelines (PUEBI), intersentential connectors like Namun are followed by a comma. So the recommended punctuation is:

Namun, keponakan perempuan saya tidak marah; dia takut badai saja. Omitting the comma is common informally but less standard.

Is the semicolon necessary? Could I use a period or a conjunction instead?

The semicolon is fine because it links two closely related independent clauses. You could also write:

Namun, keponakan perempuan saya tidak marah. Dia takut badai saja.
Keponakan perempuan saya tidak marah, melainkan takut badai. (using melainkan after a negation) All are acceptable; the choice is stylistic.

Why is the possessive saya at the end in keponakan perempuan saya?

In Indonesian noun phrases, the possessor typically follows the noun:

head noun + descriptor(s) + possessor Here, keponakan (niece/nephew) is the head, perempuan (female) narrows it, and saya marks possession: “my female niece.” You can also use enclitics:
keponakan perempuanku (= my female niece, informal) Don’t say saya keponakan perempuan for possession—that’s ungrammatical.

Is perempuan functioning like an adjective here? Could I use wanita or cewek?

Yes—perempuan (woman/female) narrows the noun like an adjective. Alternatives:

wanita: formal, often for adult females; keponakan wanita is grammatically fine but can feel odd if the niece is a child.
cewek: slang/informal (“girl/chick”); ponakan cewek is colloquial. Neutral and safe: perempuan. For male, use laki-laki or pria (pria is more formal).

Do I have to say perempuan to mean niece? Doesn’t keponakan already mean “niece”?

Keponakan is gender-neutral: niece or nephew. If gender matters, add perempuan (female) or laki-laki (male). If context already makes it clear, you can just say keponakan saya.

Why is it tidak marah and not bukan marah?

Use tidak to negate verbs and adjectives; marah here is an adjective (“angry”), so tidak is correct: tidak marah. Bukan negates nouns/pronouns. A contrasting pattern you might hear is:

Bukannya marah, dia cuma takut badai. (“Not that she was angry; she was just afraid of the storm.”)

What does saja add in dia takut badai saja?

Saja means only/just/merely. It downplays the situation: she wasn’t angry—she was “just” afraid. Synonyms:

hanya (neutral-formal): Dia hanya takut badai.
cuma (informal): Dia cuma takut badai.
doang (very informal/colloquial): Dia takut badai doang.

Note: Dia takut badai saja can, in some contexts, be read as “she’s afraid only of storms,” but here the contrast with “tidak marah” makes the “just” (not angry, only afraid) reading the natural one. To avoid any hint of ambiguity, put the limiter before the predicate: Dia hanya/cuma takut (pada) badai.

Can I place saja elsewhere?

Common, natural options:

Dia hanya/cuma takut (pada) badai. (preferred if you want “just” to clearly scope over the whole predicate)
Dia takut badai saja. (very common; slight potential for “only storms” reading, usually resolved by context) Avoid putting saja before the verb (e.g., saja dia takut badai), which is unnatural.

Does takut need a preposition? Should it be takut badai or takut pada/akan badai?

All are possible, with register nuances:

takut badai: very common and natural in everyday Indonesian.
takut pada badai: also common; slightly more explicit.
takut akan badai: more formal/literary. Colloquial speech may use takut sama badai (informal).

What exactly does badai mean? Is it thunder, lightning, hurricane…?

Badai = storm (general). Some related words:

angin ribut/angin kencang: windstorm/strong wind
hujan deras: heavy rain
petir/guntur: thunder; kilat: lightning
topan/taifun: typhoon; angin topan: cyclone/typhoon
badai salju: snowstorm; badai pasir: sandstorm

Why use dia and not ia or beliau?

dia: everyday 3rd-person singular (he/she); can be subject or object.
ia: more literary; typically used as subject only. You could write Ia takut badai saja, but it feels more written/formal.
beliau: respectful “he/she” for elders/high-status people; not used for a child/niece in normal contexts.

What’s the difference between keponakan and sepupu?

keponakan: niece/nephew (child of your sibling—or often also your spouse’s sibling’s child).
sepupu: cousin. Learners often mix these up; remember sepupu ≠ niece/nephew.

Is there a more colloquial way to say the sentence?

Yes, for casual speech:

Tapi ponakanku nggak marah; dia cuma takut badai. If you need to keep the gender:
Tapi ponakan perempuanku nggak marah; dia cuma takut badai. (Using tapi and nggak/cuma makes it informal.)