Learn Hardcore Indonesian: Kos yang mana yang perabotnya lengkap dan dekat kampus? - Which boarding room has complete furniture and is near the campus?

The first bold yang is part of the fixed interrogative phrase bold yang mana (which one).
The second bold yang starts a relative clause that describes bold kos: bold yang perabotnya lengkap dan dekat kampus (that has complete furniture and is near campus).

So the skeleton is: bold Kos yang mana [yang + description] …

Can I say bold Kos mana yang … instead of bold Kos yang mana yang …? Which sounds more natural?

Yes. Both are acceptable.

bold Kos mana yang perabotnya lengkap dan dekat kampus? → a bit tidier in careful or written Indonesian.
bold Kos yang mana yang perabotnya lengkap dan dekat kampus? → very common in everyday speech.

Functionally they mean the same thing.

Can I drop the second bold yang?

No. You need that second bold yang to introduce the descriptive clause. These are fine:

bold Kos mana yang perabotnya lengkap dan dekat kampus?
bold Kos yang mana yang perabotnya lengkap dan dekat kampus?

This is awkward/ungrammatical:

bold ✗ Kos yang mana perabotnya lengkap dan dekat kampus? (missing the relative marker before the clause)

What does the suffix bold -nya in bold perabotnya mean here? Could I omit it?

bold -nya can mean:

possessive: its/his/her
definiteness: the

Here it’s “its” (the kos’s furniture). If you remove bold -nya, bold perabot lengkap sounds odd as a sentence predicate. Natural options include:

bold perabotnya lengkap (its furniture is complete)
bold sudah ada perabotnya (it already has furniture)
bold berperabot lengkap (has complete furniture; more formal/literary)

What’s the difference between bold perabot, bold perabotan, and bold furnitur?

bold perabot = furniture (neutral, common).
bold perabotan = furnishings/household goods; often in store names, but people also say bold perabotannya lengkap in speech.
bold furnitur = “furniture” (loanword), common in ads.

All three can be understood; choose by context and tone.

Is bold lengkap the best word for “fully furnished”? What are alternatives?

Yes, bold lengkap is natural. Alternatives:

bold komplet/komplet (informal loan) = complete.
bold sudah berperabot / sudah ada semua perabotnya.
In ads you’ll see bold fully furnished (loan phrase). Avoid bold penuh (that means “full/crowded,” not “fully furnished”).

Why is there no “is” in bold perabotnya lengkap or bold dekat kampus?

Indonesian often drops a copula. Adjectives and location phrases can function as predicates:

bold Perabotnya lengkap. (Its furniture is complete.)
bold Kos itu dekat kampus. (That boarding house is near campus.) bold adalah is used mainly before nouns or for emphasis, not needed here.

Should bold dekat be followed by bold dengan? Is bold dekat ke correct?

bold dekat kampus = very common and fine.
bold dekat dengan kampus = also correct, a bit more formal.
bold dekat ke … = avoid; it’s nonstandard with bold dekat. Synonymous rephrasing: bold tidak jauh dari kampus (not far from campus).

Why not bold di before bold kampus? What’s the difference between bold dekat kampus and bold di dekat kampus?

bold (X) dekat kampus = X is near the campus (predicate of X).
bold (X) di dekat kampus = X is at a place near the campus (locative phrase). Both can work, but in your sentence the predicate pattern bold … dekat kampus is the straightforward choice. A very formal parallel version would be bold … yang … dan berada di dekat kampus.

Does bold dekat kampus mean “near a campus” or “near the campus”? How do I make it specific?

It’s context-dependent. To be specific:

Name it: bold dekat Kampus UI.
Use bold kampusnya if the campus is known in the conversation: bold dekat kampusnya.
Or refer back: bold dekat kampus yang tadi kita bahas.

What exactly is bold kos? How is it different from bold kost or bold kos-kosan?

bold kos = a rented room/boarding house (typical for students/workers), paid monthly; shared facilities are common. This is the standard spelling.
bold kost = same word, very common alternative spelling.
bold kos-kosan = a complex/row of kos rooms; also used loosely for the concept of boarding. Related terms: bold asrama (dormitory), bold kontrakan (rented house), bold apartemen (apartment).

Is it okay to mix bold perabotnya lengkap and bold dekat kampus in the same bold yang-clause even though the subjects feel different?

Yes. In Indonesian it’s fine to bundle multiple properties of the head noun inside one bold yang-clause. If you want stricter parallelism (useful in formal writing), try:

bold yang berperabot lengkap dan berlokasi dekat kampus
bold yang perabotannya lengkap dan lokasinya dekat kampus

Are there other natural word orders?

Yes:

bold Kos mana yang perabotnya lengkap dan dekat kampus? (tidy)
bold Kos yang perabotnya lengkap dan dekat kampus itu yang mana? (puts the description first, then asks “which one?”)
bold Yang perabotnya lengkap dan dekat kampus, kos yang mana? (fronted description for emphasis)

How do I ask about multiple options (which ones)?

Use bold mana saja or a reduplicated noun:

bold Kos mana saja yang perabotnya lengkap dan dekat kampus?
bold Kos-kos mana yang perabotnya lengkap dan dekat kampus?

What would a casual version sound like?

Colloquial tweaks:

bold Kost yang mana yang perabotnya lengkap dan deket kampus?
bold Kost mana yang furniturnya lengkap dan deket kampus? Softening particles: add bold ya or bold sih at the end: bold … deket kampus ya?

How might someone answer this question in Indonesian?

Examples:

bold Coba Kos Mawar; perabotannya lengkap dan dekat dengan kampus.
bold Yang di Jalan Kenanga. Sudah fully furnished dan cuma 5 menit jalan kaki ke kampus.
bold Kos Pelangi; furniturnya lengkap kok, dan letaknya dekat kampus.