Learn Hardcore Indonesian: Kami punya rencana cadangan seandainya rapat ditunda. - We have a backup plan in case the meeting is postponed.

kami = we (excluding the person you’re talking to).
kita = we (including the person you’re talking to). Use kami if the listener is not part of the group that has the plan. If the listener is included, say: Kita punya rencana cadangan seandainya rapat ditunda.

Is punya the best verb for “to have” here? What about mempunyai, memiliki, or ada?

punya: very common and neutral in everyday Indonesian. Perfect here.
mempunyai: a bit more formal; interchangeable with punya.
memiliki: formal and often preferred in writing. Example: Kami memiliki rencana cadangan…
ada: existential “there is/are.” Colloquially some say Kami ada rencana cadangan, but in standard Indonesian ada is better used without a subject: Ada rencana cadangan (There is a backup plan).

Why is there no article like “a” before rencana?

Indonesian has no articles (no “a/an/the”). rencana cadangan can mean “a backup plan” or “the backup plan,” depending on context. You can add sebuah to emphasize “one”: Kami punya sebuah rencana cadangan, but it’s usually unnecessary.

Is the noun order rencana cadangan correct? Why not cadangan rencana?

Yes. In Indonesian noun–noun compounds, the head noun typically comes first and the modifier second:

rencana (plan)
- cadangan (reserve/backup) → “backup plan.” Saying cadangan rencana would mean “a reserve of plans,” which is not the intended meaning.

What exactly does cadangan mean? Any alternatives?

cadangan = reserve, spare, backup (general-purpose). Examples: ban cadangan (spare tire), daya cadangan (backup power), rencana cadangan (backup plan).
Alternatives:
- rencana alternatif (an alternative plan)
- rencana B (Plan B; casual, borrowed from English)
- rencana pengganti (replacement plan)

How does seandainya differ from jika, kalau, apabila, and andaikan/andai?

seandainya: “if/supposing,” often more hypothetical; can feel like “in case.” Also used in counterfactual/regret contexts.
kalau: very common in speech; general “if/when.”
jika: neutral and a bit more formal; common in writing.
apabila: formal; interchangeable with jika.
andaikan/andai: “if only/supposing,” often more wistful or counterfactual.

Your sentence would also work with kalau/jika/apabila.

Can I put the seandainya clause first?

Yes:

Seandainya rapat ditunda, kami punya rencana cadangan. When the conditional clause comes first, use a comma after it.

Why is there no comma before seandainya rapat ditunda in the original?

In Indonesian, you typically don’t put a comma before a subordinate clause that follows the main clause. You do use a comma when the subordinate clause comes first:

Seandainya rapat ditunda, kami…

What’s going on with rapat ditunda grammatically?

ditunda is passive voice with the prefix di- on the verb tunda (postpone).
Agentless passive: Rapat ditunda (The meeting is postponed).
If you want to name the agent: Rapat ditunda oleh panitia (by the committee).
Active voice: Panitia menunda rapat (The committee postpones the meeting).

Is it ditunda or di tunda?

It must be one word: ditunda. The prefix di- attaches to verbs to form passive. The separate word di as a preposition (at/in/on) is written separately: di kantor, di rumah.

What’s the difference between ditunda, diundur, tertunda/terunda, and dibatalkan?

ditunda: postponed (by someone); neutral and standard.
diundur: pushed back/moved later; very common in everyday speech.
tertunda/terunda: ended up being postponed/delayed (state/result, sometimes unintentional).
dibatalkan: canceled (no longer happening).
Also useful: dijadwal ulang (rescheduled).

Should it be rapat, rapatnya, or rapat tersebut?

rapat: “the meeting” in general; definiteness is inferred from context.
rapatnya: that specific/previously known meeting (the meeting in question).
rapat tersebut: “that meeting” (explicit, formal). All are possible depending on how specific/formal you want to be.

Do I need akan to mark the future in the if-clause?

No. Indonesian usually doesn’t mark future in the conditional clause. Say:

Jika/kalau/seandainya rapat ditunda, kami… You can add akan in the main clause to stress a future response:
… kami akan menjalankan rencana cadangan.

Is there a more formal version of the whole sentence?

Yes, for formal writing:

Kami memiliki rencana cadangan apabila rapat ditunda. Or with preparation emphasized:
Kami telah menyiapkan rencana cadangan untuk berjaga-jaga apabila rapat ditunda.

How do I negate or make a question?

Negation: Kami tidak punya rencana cadangan.
Yes–no question (neutral): Apakah kami/kita punya rencana cadangan?
Casual: Kita punya rencana cadangan? / Ada rencana cadangan gak?