Learn Hardcore Indonesian: Debu tebal ada di bawah sofa; kami menyapu dan mengepel secara rutin. - Thick dust is under the sofa; we sweep and mop routinely.

ada marks existence/presence (there is/are) or location. It turns the phrase into a full clause.
Without ada, Debu tebal di bawah sofa reads like a noun phrase (thick dust under the sofa), not a full sentence, unless you’re writing a note/headline.
Most natural neutral option: Ada debu tebal di bawah sofa.

Is Debu tebal ada di bawah sofa the best word order? What other natural orders are there?

All are grammatical, with different emphasis:

Neutral/existential: Ada debu tebal di bawah sofa.
Topicalizing the dust (what we’re talking about): Debu tebal ada di bawah sofa.
Emphasizing location: Di bawah sofa ada debu tebal.

Why not use adalah instead of ada?

adalah links two nouns (X = Y): Ini adalah masalah besar.
It is not used for existence or before a prepositional phrase. So Debu tebal adalah di bawah sofa is ungrammatical.
For location/existence, use ada (or more formal berada: Debu tebal berada di bawah sofa).

Why is it di bawah (two words) and not dibawah?

When di is a preposition (in/at/on), it’s written separately: di bawah, di rumah, di meja.
When di- is the passive verb prefix, it attaches to the verb: ditulis, dibersihkan.
Here di is a preposition, so it must be separate: di bawah.

Could I say di kolong sofa instead of di bawah sofa?

Yes. di kolong means in the space under something (sofa/bed/table) and often sounds more specific/idiomatic for dust:

Ada debu tebal di kolong sofa. Both di bawah and di kolong are fine; choose based on how specific you want to be.

Why kami and not kita?

kami = we (excluding the person spoken to).
kita = we (including the person spoken to). Use kami if the listener isn’t part of the cleaning crew; use kita if you include them.

Why are the verbs menyapu and mengepel (not just sapu or pel)?

sapu (broom) and pel (mop) are nouns. Verbs are formed with the meN- prefix:
- menyapu = to sweep (from sapu). Rule: meN- + s → meny- and the s drops.
- mengepel = to mop (from pel). Monosyllabic roots often take menge- (cf. mengecat, mengebor).
Colloquial speech often uses nyapu and ngepel.

Do I need to mention the object, like lantai (floor)?

You can omit the object when it’s obvious from context: kami menyapu dan mengepel is fine.
If you want to be explicit: Kami menyapu dan mengepel lantai secara rutin.

Is secara rutin necessary? Can I just say rutin? What are alternatives?

secara rutin is a more formal adverbial phrase; everyday speech often just uses rutin:
- Kami rutin menyapu dan mengepel.
- Kami menyapu dan mengepel secara rutin.
- Kami secara rutin menyapu dan mengepel.
Alternatives: secara teratur (regularly), sering (often, but not necessarily on a schedule), biasanya (usually), setiap minggu/hari (every week/day).

Is the semicolon natural in Indonesian? What else could I use?

Semicolons are used similarly to English in formal writing, but in everyday Indonesian a period or a comma + connector is more common.
Options:
- Period: … sofa. Kami menyapu …
- Contrast: …, namun/padahal kami menyapu dan mengepel secara rutin.
- Cause: …, jadi kami menyapu dan mengepel secara rutin.

How do I make the contrast explicit, like “even though we clean regularly”?

Use a contrastive linker:

Padahal kami rutin menyapu dan mengepel, ada debu tebal di kolong sofa.
Meskipun kami rutin menyapu dan mengepel, tetap ada debu tebal di kolong sofa.
Walaupun kami …, … works the same as meskipun.

How do tenses work here? How would I say it in the past, present, or future?

Indonesian has no verb tense; use time/aspect words:

Past: Tadi/kemarin ada debu tebal …, padahal kami sudah menyapu dan mengepel.
Progressive: Sekarang ada debu tebal …, padahal kami sedang menyapu dan mengepel.
Future: Nanti kalau ada debu tebal …, kami akan menyapu dan mengepel.
Habitual: Biasanya kami menyapu dan mengepel setiap minggu.

Is debu tebal the only way to say this? What about banyak debu or berdebu?