Learn Hardcore Indonesian: Saya belajar di perpustakaan ketimbang di kafe. - I study in the library rather than at the cafe.

Register: ketimbang is more colloquial; daripada is neutral and widely used in formal contexts.
Your sentence could be: Saya belajar di perpustakaan daripada di kafe.

Is the sentence okay without adding lebih (e.g., “lebih suka/lebih sering”)?

Yes. Ketimbang (and daripada) can directly contrast two parallel options without an explicit “more.” If you want to state the basis of preference, you can add it:

Preference: Saya lebih suka belajar di perpustakaan daripada di kafe.
Frequency: Saya lebih sering belajar di perpustakaan daripada di kafe. The original sentence already reads naturally as a statement of preference/habit.

Can I put the contrast first, like “Rather than at the cafe, I study at the library”?

Yes: Ketimbang di kafe, saya belajar di perpustakaan.
This fronting is fine and somewhat more written/formal in feel. You can also do it with daripada: Daripada di kafe, saya belajar di perpustakaan.

Do I need to repeat di before kafe, or can I say “ketimbang kafe”?

Best practice is to repeat di: … ketimbang di kafe.
Dropping it (… ketimbang kafe) occurs in casual speech but can sound like you’re comparing “the library” to “the cafe” as entities rather than contrasting locations. Keep di for clarity and correctness.

What’s the difference between di, ke, and dari?

di = at/in/on (static location): di perpustakaan “at the library”
ke = to/toward (destination): ke perpustakaan “to the library”
dari = from (origin): dari perpustakaan “from the library” Your sentence uses di because it states where the studying happens.

Why is di written separately here? Sometimes I see di- attached to words.

di (separate) is a preposition meaning “at/in/on”: di kafe, di sekolah.
di- (attached) is a passive verb prefix: dibaca “(is) read,” ditulis “(is) written.” Never write the preposition as one word with the noun (✗ dikafe).

How do I mark tense/aspect? Is this present, habitual, or something else?

Indonesian doesn’t inflect verbs for tense. Context and time words do the work:

Present progressive: Saya sedang belajar di perpustakaan …
Habitual: Saya biasanya belajar di perpustakaan …
Past (earlier today): Tadi saya belajar di perpustakaan …
Future: Besok saya akan belajar di perpustakaan …

How do I say “at the library” vs “at a library” or a specific place?

Indonesian has no articles. Use modifiers:

Generic: di perpustakaan (could be “at a/the library”)
Specific: di perpustakaan itu (“at that library”), di perpustakaan kampus (“at the campus library”)
Indefinite, if needed: di sebuah perpustakaan (“at a library”) Same for kafe: di kafe itu, di sebuah kafe.

Is Saya the only way to say “I”? What about aku or gue?

saya: polite/neutral, safe with strangers and in formal contexts.
aku: informal/intimate; common among friends/family, in songs, etc.
gue/gua: very informal, Jakarta colloquial. All work grammar-wise; choose based on formality and audience.

Can I use other “instead of” words like alih-alih or dibanding(kan) (dengan)?

alih-alih = “instead of (doing X, do Y),” often with a corrective tone:
Saya belajar di perpustakaan alih-alih di kafe.
dibandingkan (dengan)/dibanding = “compared with/to,” more formal and often paired with a comparative like lebih:
Belajar di perpustakaan lebih tenang dibanding (belajar) di kafe. For a simple “rather than,” daripada or ketimbang is most straightforward.

What’s the difference between belajar and mempelajari? Can I say “mempelajari di perpustakaan”?

belajar = to study/learn (intransitive); can take a place with di:
Saya belajar di perpustakaan.
mempelajari = to study/examine (transitive; needs an object):
Saya mempelajari biologi di perpustakaan. Don’t say ✗ mempelajari di perpustakaan without an object.

Any pronunciation tips for these words?

Saya: SA-ya (a as in “father”)
belajar: bə-LA-jar (first e is a schwa, “uh”)
di: dee
perpustakaan: pər-pus-ta-ka-an (schwa on the first e; the last “-an” is its own syllable)
ketimbang: kə-tim-bang (final ng is the velar nasal as in “sing”)
kafe: KA-fe (no accent mark in standard Indonesian) Indonesian “r” is tapped/trilled.

Is kafe the same as “cafe”? Are there other terms I might hear?

Standard Indonesian uses kafe (no accent). You may see English “cafe” in signage, but kafe is the Indonesian spelling.
Related places: warung (small eatery), kedai kopi (coffee shop), restoran (restaurant). Use the one that matches the setting.

Could I say “It’s better to study at the library than at the cafe”?

Yes:

Neutral: Lebih baik belajar di perpustakaan daripada di kafe.
Colloquial: Mending belajar di perpustakaan daripada di kafe. These constructions express an evaluative “better to … than …,” not just a neutral preference.