Learn Hardcore Indonesian: Dia menempel stiker hijau pada aksesori yang sudah diperiksa. - He/She sticks a green sticker on the accessory that has already been checked.

dia is gender‑neutral and can mean either “he” or “she.” It’s the most common 3rd‑person singular pronoun and can be used as subject or object.
ia is a bit more formal/literary and is normally used only as a subject (rarely as an object).
beliau is a respectful form used for elders, officials, teachers, etc. (roughly “he/she” with respect).

What’s the difference between menempel and menempelkan? Which fits best here?

menempel can mean “to stick / be stuck (on),” and in everyday speech many speakers also use it transitively (“to stick something”).
menempelkan is the clearly transitive, causative form: “to stick/attach something (onto something).”
In careful or formal Indonesian, prefer menempelkan stiker … (pada/di …) for “stick a sticker onto ….” The sentence with menempel is very common in speech and informal writing and will be readily understood.

Can menempel be used intransitively to mean “to be stuck”?

Yes. Examples:

Stiker itu menempel di aksesori. = “The sticker is stuck on the accessory.”
Transitive counterpart: Dia menempelkan stiker itu di/pada aksesori. = “He/She sticks the sticker onto the accessory.”

Why use the preposition pada here? Could I use di or ke instead?

pada often marks the target of an action in more neutral/formal style: menempel(kan) … pada ….
di marks the resulting location: menempel(kan) … di … is also very common.
ke marks direction (“to/onto”): menempelkan … ke … is heard in speech; some editors still prefer pada/di.
Don’t use kepada here (that’s for recipients, usually people).

What does yang do in this sentence? Can I drop it?

yang introduces a relative clause and links it to the noun it modifies. Here it marks “the accessory that has already been inspected.”
You cannot omit yang in such clauses. Without it, the sentence would be ungrammatical or unclear.

How does sudah diperiksa work? What are sudah and the di- prefix doing?

sudah marks completed aspect (“already”).
diperiksa is the passive form: di- (passive prefix) + the verb periksa (“examine/inspect”) → “be/been inspected.”
Together, sudah diperiksa = “has already been inspected.”

How do I mention who inspected the accessory?

Several options:

Passive with an agent: … yang sudah diperiksa oleh petugas.
Passive with clitic agent: … yang sudah diperiksanya. (“… inspected by him/her.”)
Active relative clause: … yang sudah dia/ia periksa. (Note: in this pattern the verb appears without the me- prefix; not memeriksa.)

Can I use an active relative clause instead of the passive one?

Yes:

Passive (given): aksesori yang sudah diperiksa (agent unspecified).
Active: aksesori yang sudah dia/ia periksa (explicit agent as subject inside the clause).
Don’t say yang sudah dia memeriksa; in object relatives the verb appears without me-: yang … dia periksa.

Does the relative clause modify aksesori or stiker hijau?

It modifies aksesori, because yang … attaches to the closest preceding noun phrase. If you wanted it to modify the sticker instead, you’d say:

Dia menempel stiker hijau yang sudah diperiksa pada aksesori.

How do I show definiteness and number (one vs many) for stiker/aksesori?

Indonesian has no articles. You can specify if needed:

One/indefinite: sebuah aksesori, sebuah stiker hijau.
Definite: aksesori itu/tersebut, stiker hijau itu.
Plural: reduplication or quantifiers, e.g., aksesori-aksesori, banyak/berbagai aksesori; beberapa stiker hijau.

Are the spellings aksesori and stiker correct? I often see aksesoris and “sticker.”

Standard Indonesian (KBBI) prefers aksesori and stiker.
aksesoris and “sticker” are common in informal contexts but are nonstandard spellings.

Why is diperiksa one word but di aksesori two words?

di- as a passive prefix attaches to a verb and is written together: diperiksa (“be inspected”).
di as a preposition meaning “at/in/on” is written separately: di aksesori.
Quick test: if you can add oleh after it (passive), it’s the prefix and should be attached (e.g., diperiksa [oleh …]).

Could I replace diperiksa with dicek or other verbs?

Yes, with register differences:

dicek (“checked”) is casual/colloquial.
diperiksa is neutral/formal.
diinspeksi (“inspected”) sounds official/technical. Choose based on context and tone.

How would I rewrite the sentence in a fully passive form with the sticker as the subject?

Stiker hijau ditempelkan pada aksesori yang sudah diperiksa (oleh dia/oleh petugas). This highlights the sticker rather than the person doing the action.

What’s a natural command/imperative version?

Neutral/polite: Tempelkan stiker hijau pada aksesori yang sudah diperiksa.
Colloquial: Tempel stiker hijau di aksesori yang sudah diperiksa.