Learn Hardcore Indonesian: Saya menunjukkan kupon diskon kepada kasir.

menunjuk = to point (at something). Example: Dia menunjuk papan tulis. (He points at the board.)
menunjukkan = to show, to point out (to someone). It typically takes a thing and a recipient. Example: Dia menunjukkan kupon diskon kepada kasir.
Formally, menunjukkan comes from the root tunjuk with the prefix meN- and the suffix -kan. The suffix -kan often has a causative/applicative sense (cause someone to see/know; direct an action toward a recipient).

Why is kepada used? Could I use ke, pada, untuk, or sama instead?

kepada: preferred in neutral/formal style for recipients (people). Your sentence is perfectly standard.
ke: literally “to (a place).” In everyday speech it’s also used for recipients: … ke kasir is very common.
pada: formal and versatile; can replace kepada with people in formal writing, but sounds stiff in casual speech.
untuk: means “for,” not “to (a person).” Don’t use it here.
sama: very colloquial for “to/with.” In casual Indonesian: … sama kasir. Examples:
Formal/neutral: Saya menunjukkan kupon diskon kepada kasir.
Conversational: Saya nunjukin kupon diskon ke/sama kasir.

Can I say Saya menunjukkan kasir kupon diskon like “I showed the cashier a coupon”?

No. Indonesian doesn’t allow a “double-object” structure here. The recipient must be introduced by a preposition:

Correct: Saya menunjukkan kupon diskon kepada/ke/sama kasir.
Incorrect: Saya menunjukkan kasir kupon diskon.

Can I change the word order?

Yes. Common variants:

Neutral active: Saya menunjukkan kupon diskon kepada kasir.
Fronted object (topicalization): Kupon diskon saya tunjukkan kepada kasir. (Note the verb becomes tunjukkan without meN- in this pattern.)
Passive: Kupon diskon ditunjukkan kepada kasir (oleh saya). (Agent with oleh is optional and often omitted.) All mean essentially the same; choose based on what you want to emphasize.

How would I say it casually in everyday speech?

Aku nunjukin kupon diskon ke/sama kasir. Notes:
nunjukin is the colloquial form of menunjukkan.
aku is casual; saya is neutral/polite.
ke/sama are more casual than kepada.

How do I turn it into a request or instruction?

Imperative: Tunjukkan kupon diskon ke/kepada kasir!
Polite request:
- Tolong tunjukkan kupon diskon ke/kepada kasir.
- Bisa/boleh tunjukkan kupon diskon ke kasir?
Very polite/service tone: Silakan tunjukkan kupon diskon kepada kasir.

Is kupon diskon natural? Are there other options?

Yes, kupon diskon is common and clear. Alternatives:

voucher diskon / voucher belanja (very common in stores).
kupon potongan / kupon promo (discount/promo coupon). Avoid karcis here; karcis means ticket (e.g., karcis parkir).

Do I need “a/the”? How do I say “the cashier,” “a cashier,” etc.?

Indonesian has no articles.

“the cashier” by context, or make it explicit: kasir itu/tersebut/kasirnya (that/the mentioned cashier).
“a cashier”: seorang kasir (one cashier).
Quantifying the coupon: satu kupon, dua kupon, or with a measure word: selembar kupon (one sheet/coupon). Examples:
Saya menunjukkan satu kupon diskon kepada kasir itu.
Saya menunjukkan kupon diskon kepada kasirnya.

How do I show past time? Does menunjukkan already mean “showed”?

Verbs don’t change for tense. Use time words or aspect markers:

Completed: sudah/telah — Saya sudah menunjukkan kupon diskon kepada kasir.
Recent: barusan/baru saja — Saya baru saja menunjukkan…
Earlier today: tadi — Tadi saya menunjukkan… Or add time adverbs: kemarin, tadi pagi, etc.

Is the spelling menunjukkan with double k correct?

Yes. Correct: menunjukkan (root tunjuk + -kan → tunjukkan; with meN- → menunjukkan).
Common typo to avoid: menunjukan (missing one k).

Could I use memperlihatkan, menyerahkan, or menyodorkan instead of menunjukkan?

memperlihatkan: to show; a bit more formal/literary. Near-synonym of menunjukkan.
- Saya memperlihatkan kupon diskon kepada kasir.
menyerahkan: to hand over/submit (you give it to them).
- Saya menyerahkan kupon diskon kepada kasir.
menyodorkan: to proffer/extend toward someone (gesture-focused).
- Saya menyodorkan kupon diskon kepada kasir. Choose based on whether you merely displayed it (menunjukkan/memperlihatkan) or actually handed it over (menyerahkan/menyodorkan).

Can I use aku instead of saya?

Yes. saya is neutral/polite; aku is casual/intimate. Regionally, some people use gue/gua (Jakarta) or aq in texting (not recommended in formal contexts).

Polite: Saya menunjukkan…
Casual: Aku nunjukin…

Can I omit the subject pronoun?

In a full sentence like this, you normally keep it. But omission happens when context is clear or in instructions:

Elliptical/contextual: Sudah menunjukkan kuponnya ke kasir? (Have you shown your coupon to the cashier?)
Instruction: Tunjukkan kupon ke kasir. (subject implied) In narrative statements, prefer keeping saya/aku.

How do I say “my/your/their discount coupon” here?

Add a possessor or -nya:

kupon diskon saya/aku (my discount coupon)
kupon diskon kamu/Anda (your)
kupon diskon mereka (their)
kupon diskonnya can mean “the/that coupon” (previously known) or “his/her coupon,” depending on context. Example: Saya menunjukkan kupon diskon saya kepada kasir.

Is kepada one word or two? What about ke pada?

kepada is one word and is the correct form here. Don’t write ke pada.
Separately, ke and pada exist as independent prepositions in other contexts, but the recipient-marking form is kepada.

Can I topicalize the recipient?

Yes, but it’s less common. For emphasis you can say:

Kepada kasir, saya menunjukkan kupon diskon. (formal/emphatic) In casual speech it’s more natural to keep the normal order or front the object instead:
Kupon diskon saya tunjukkan kepada kasir.