Learn Hardcore Indonesian: Kami mengisi ulang kartu bus secepat mungkin di stasiun. - We top up the bus card as fast as possible at the station.

kami = we (excluding the listener).
kita = we (including the listener).
Use kita if you mean “you and I/we all”: Kita mengisi ulang kartu bus…

How is tense shown? Is this past, present, or future?

Indonesian usually does not mark tense. Context or time words do:

Past: tadi, kemarin, or sudah (already) → Kami sudah mengisi ulang…
Present/progressive: sedang → Kami sedang mengisi ulang…
Future: nanti, besok, or akan → Kami akan mengisi ulang…

What’s the difference between mengisi ulang, isi ulang, top up, and mengisi kembali?

mengisi ulang = to refill/recharge (verb phrase).
isi ulang = a noun/label or imperative-like phrase (seen on signs): Isi ulang di sini.
top up = borrowed, very common in speech: top up kartu; neutral but informal.
mengisi kembali = more formal/literary “refill again.”
All can mean “to top up,” but mengisi ulang is a safe, neutral choice.

Can mengisi ulang be split, like mengisi kartu bus ulang?

Normally keep it together before the object: mengisi ulang kartu bus.
Putting ulang after the object sounds odd. If you want “again” after the object, use lagi: mengisi kartu bus lagi (more casual).

Does kartu bus mean one card or several? How do I show plural or possession?

Indonesian doesn’t mark number by default.

Could be one or many from context.
Plural: kartu-kartu bus, beberapa kartu bus, dua kartu bus.
Possession: kartu bus kami (our bus card[s]); kartu busnya (the/his/her bus card; context decides which).

Is kartu bus the natural phrase? What about tiket bus or kartu transport?

kartu bus is understandable (a transit/e-money card for buses).
tiket bus = a one-ride ticket.
You’ll also hear kartu transport, kartu e-money, or brand names (e.g., Flazz, Brizzi) depending on the city/system.

Why bus and not bis?

Standard modern spelling is bus (per KBBI). bis is older/colloquial; still heard in speech, but write bus.

Why di stasiun and not ke stasiun? Can I use pada?

di = at/in (location): di stasiun.
ke = to/toward (movement): ke stasiun.
pada is formal and used with abstract nouns or pronouns, not with plain place nouns here; use di.

Is stasiun right for buses? Should it be halte or terminal?

stasiun = station (usually trains/MRT/LRT).
halte = bus stop.
terminal = bus terminal/depot.
If you literally mean a bus stop, say di halte; for intercity bus terminal, di terminal. di stasiun is fine if you mean a rail station where you can top up the card.

Where can secepat mungkin go? Can I move it?

Natural positions:

Kami secepat mungkin mengisi ulang kartu bus di stasiun.
Kami mengisi ulang kartu bus di stasiun secepat mungkin. Both are fine; earlier placement emphasizes urgency; end placement sounds a bit more neutral. Sentence-initial (Secepat mungkin, …) is formal/written style.

Difference between secepat mungkin, secepatnya, sesegera mungkin, and segera?

secepat mungkin = as fast/soon as possible.
secepatnya = ASAP; directive, often in requests/instructions.
sesegera mungkin = as soon as possible (focus on time, near-synonym of secepat mungkin).
segera = immediately/soon (not “as … as possible”).

What does the se- in secepat do?

It forms “as … as” with adjectives, especially in the pattern se- + adjective + mungkin:

secepat mungkin (as fast as possible)
setinggi mungkin, sebanyak mungkin, serendah mungkin, sepelan mungkin

Is mungkin here “maybe”?

No. In se… mungkin, mungkin means “possible.”

Mungkin dia datang. = Maybe he’ll come.
Secepat mungkin. = As fast as possible.

How do I say we already/will/are (right now) topping up?

Already: Kami sudah mengisi ulang…
Right now: Kami sedang mengisi ulang…
Will: Kami akan mengisi ulang… or Nanti kami mengisi ulang…

How do I negate this correctly? tidak or bukan?

Use tidak with verbs/adjectives: Kami tidak mengisi ulang kartu bus…
Use bukan with nouns/pronouns or for corrective contrast: Itu bukan kartu bus. / Kami bukan mengisi ulang, tetapi membeli kartu baru.

Can I use mengisi lagi instead of mengisi ulang?

Yes, mengisi lagi = fill again (more casual). mengisi ulang is standard/neutral, common in service contexts. Both are acceptable; avoid doubling: mengisi ulang… lagi.

Should mengisi ulang or isi ulang have a hyphen?

No hyphen. Write mengisi ulang and isi ulang as two words. The nominalized form is pengisian ulang.

Do I need an article like “the/a” for stasiun? How do I specify?

Indonesian has no articles. Use modifiers when needed:

Specific: di stasiun itu/ini
Indefinite: di sebuah stasiun
More specific: di stasiun kereta, di stasiun MRT

Any pronunciation tips for this sentence?

c = “ch” (English “chop”): cepat → “cheh-paht.”
u = “oo”: bus, ulang, kartu.
ng = “ng” in “sing”: mengisi.
stasiun = “sta-si-un” (three syllables).
Stress is fairly even; don’t over-stress one syllable like in English.