Learn Hardcore Indonesian: Kupon kedaluwarsa itu ditempel stiker agar kasir tahu. - That expired coupon is given a sticker so the cashier knows.

In Indonesian, demonstratives like itu come after the noun phrase they modify. Kupon kedaluwarsa itu means “that/the expired coupon.” If you move itu, the meaning/structure changes:

Kupon itu kedaluwarsa = “That coupon is expired” (a full sentence).
Itu kupon kedaluwarsa = “That is an expired coupon” (equative sentence).

Could I say Kupon yang kedaluwarsa itu instead? What’s the difference?

Yes. Kupon yang kedaluwarsa itu is also correct. Adding yang can sound a bit more explicit or careful, like “the coupon that is expired.” Without yang, kedaluwarsa functions as a simple adjective: kupon kedaluwarsa = “expired coupon.” Both are natural; yang is often used when the modifier is longer or needs emphasis/clarity.

Is kedaluwarsa the correct spelling? I’ve seen kadaluwarsa too.

Both appear in real life, but the recommended standard form is kedaluwarsa. You will also see kadaluwarsa widely in everyday writing. Meaning is the same: “expired.” The word can act like an adjective or a predicate:

Adjective: kupon kedaluwarsa
Predicate: kupon itu sudah kedaluwarsa = “the coupon has expired.”

Who is doing the action in ditempel stiker? There’s no agent.

It’s an agentless passive. The subject is the patient (kupon kedaluwarsa itu), and the agent (the person who sticks the sticker on) is left unspecified. If you want to mention the agent, add oleh:

Kupon kedaluwarsa itu ditempeli stiker oleh petugas. = “The expired coupon was marked with a sticker by a staff member.”

Is ditempel stiker fully grammatical, or should it be ditempeli/dipasangi?

You’ll hear and read ditempel stiker quite often, especially in neutral–informal Indonesian. In more careful/formal style, many prefer:

ditempeli stiker or dipasangi stiker (coupon as the location/recipient)
Or restructure with the sticker as subject: Stiker ditempel pada kupon (yang) kedaluwarsa itu. Avoid Kupon … ditempelkan stiker; with -kan, the natural passive is Stiker ditempelkan pada kupon …

What’s the difference among tempel / menempel / menempelkan / menempeli / ditempel / ditempeli?

menempel: intransitive “to stick/adhere” (subject sticks). Example: Stiker itu menempel di kupon.
menempelkan X pada Y: causative; paste X onto Y. Example: Mereka menempelkan stiker pada kupon.
menempeli Y (dengan) X: Y is the location/recipient. Example: Mereka menempeli kupon dengan stiker.
ditempel: passive of tempel; often heard with an instrument NP in speech: Kupon ditempel stiker.
ditempeli: passive of menempeli (location passive). Example: Kupon itu ditempeli stiker. Colloquial Jakarta speech also uses ditempelin for ditempeli.

Do I need a preposition like dengan or pada here?

It depends on the structure:

Location passive (coupon as recipient): Kupon itu ditempeli stiker (no preposition is fine). You can add dengan: ditempeli dengan stiker, but many drop it.
Sticker as subject: Stiker ditempel pada/di kupon. pada is more formal; di is very common in speech.

What does agar mean, and how is it different from supaya, biar, and untuk?

agar and supaya: “so that / in order that,” followed by a clause. agar is a bit more formal; supaya is neutral.
biar: informal equivalent of supaya.
untuk: “for / to,” used with a noun or a verb phrase (often a verbal noun). For a purpose clause with a subject, use agar/supaya/biar, not untuk. Compare:
- Kupon … ditempeli stiker agar/supaya kasir tahu.
- Kupon … ditempeli stiker untuk memberi tahu kasir. (uses a verb phrase after untuk)

Why is it tahu instead of mengetahui?

tahu is the everyday verb “to know.” mengetahui is more formal and typically transitive; it fits best when you specify what is known, often with bahwa:

Natural: agar kasir tahu (“so the cashier knows”).
More formal/explicit: agar kasir mengetahui bahwa kupon itu kedaluwarsa (“so the cashier knows that the coupon is expired”).

Does kasir mean “the cashier,” “a cashier,” or “cashiers”?

Without markers, kasir is number-neutral and (in English terms) article-less; context decides. It often reads as “the cashier” in a store context. To be explicit:

Specific/possessive feel: kasirnya (“the cashier [of this place]/that particular cashier”)
Plural: para kasir or kasir-kasir

Is stiker singular or plural here?

Indonesian doesn’t mark number by default. stiker could be one or more. To specify:

One: sebuah stiker
Some: beberapa stiker
Plural in general: stiker-stiker

What tense is this—“was” or “is” pasted?

Indonesian doesn’t inflect for tense. ditempel can be past or present depending on context. You can mark it if needed:

Past/completed: sudah/telah → Kupon itu sudah ditempeli stiker.
Future: akan → Kupon itu akan ditempeli stiker.
Recent: barusan/tadi

Can I move the agar-clause to the beginning?

Yes. Purpose clauses with agar/supaya/biar can go first:

Agar kasir tahu, kupon kedaluwarsa itu ditempeli stiker. In writing, a comma after the fronted purpose clause is common; in speech, intonation does the job.

How would you say this in active voice?

Several natural options:

With the sticker as object: Mereka menempelkan stiker pada kupon kedaluwarsa itu agar kasir tahu.
With the coupon as object (location-object pattern): Mereka menempeli kupon kedaluwarsa itu dengan stiker agar kasir tahu.
Sticker as subject in passive (alternate passive): Stiker ditempel pada kupon kedaluwarsa itu agar kasir tahu.