Learn Hardcore Indonesian: Dia menyimpan pulpen di saku kemejanya, lalu saya merapikan dasinya. - He/She keeps a pen in his/her shirt pocket, then I straighten his/her tie.

dia = he/she (gender-neutral), singular, usually for people.
ia = he/she, more formal/literary; mostly used as a subject. Example: Ia pergi. Not natural as an object.
beliau = respectful he/she (for elders, respected figures).
Plural is mereka (they). Indonesian doesn’t mark gender.

What does the suffix -nya mean in kemejanya and dasinya?

-nya most often marks third-person possession: kemejanya = his/her shirt; dasinya = his/her tie.
It can also mark definiteness (roughly the): mejanya = the table (already known in context).
In this sentence, both kemejanya and dasinya naturally refer back to the same previously mentioned third person.

How do I know whose tie dasinya refers to? Could it mean my tie?

-nya never refers to the speaker or the listener. For those, use -ku / saya (my) and -mu / kamu (your).
Here, dasinya is almost certainly the tie of dia mentioned earlier, unless the broader context points to another third person.
To be extra explicit, say dasi dia, or use a name: dasi Budi.

Why is it di saku and not ke saku?

di marks a location/state: at/in a place. di saku = in the pocket.
ke marks movement toward: to a place. You’d use it if the verb itself expresses insertion.
Natural options:
- menyimpan/menaruh X di saku (put/store X in the pocket)
- memasukkan X ke (dalam) saku (put/insert X into the pocket)

Is menyimpan the best verb here? How is it different from menaruh or memasukkan?

menyimpan = to keep/store/put away (often with the idea of safekeeping or for later).
menaruh = to put/place (neutral, very common in speech).
memasukkan = to put/insert something into something.
All can fit, but nuances differ:
- Everyday speech: menaruh pulpen di saku.
- Emphasize insertion: memasukkan pulpen ke dalam saku.
- Emphasize keeping it safe: menyimpan pulpen di saku.

What exactly does merapikan mean? Could I use meluruskan or membetulkan instead?

merapikan (from rapi) = to make tidy, straighten up, neaten. Works for hair, clothes, beds, documents, etc.
meluruskan = to make straight (physically or figuratively clarify). For a tie, it focuses on straightness.
membetulkan = to correct/fix an error. For a tie it can work, but it suggests something was wrong.
For a tie, merapikan dasi is the most neutral and common.

Why is it saku kemejanya and not saku dari kemejanya?

Indonesian commonly uses noun–noun compounds for possession/type: saku kemeja = shirt pocket; saku celana = pants pocket.
dari for possession is usually avoided here; saku dari kemejanya sounds unnatural.
To say his/her pocket without specifying the garment: sakunya.

Can I write kemeja dia or dasi dia instead of kemejanya and dasinya?

Yes. kemeja dia / dasi dia are fully correct and slightly more explicit about the possessor. The -nya forms (kemejanya / dasinya) are compact and very common in natural speech.

Is the comma before lalu required? Can I use kemudian or terus?

The comma is optional. Both Dia …, lalu saya … and Dia … lalu saya … are fine.
Alternatives:
- kemudian = then/after that (neutral, a bit more formal).
- terus = then/and then (informal/colloquial).
- setelah itu / habis itu = after that.

What’s the difference between di (separate) and the prefix di- (no space)?

di (separate) = preposition meaning at/in/on: di saku, di meja.
di- (attached) = passive prefix on verbs: disimpan (is/was kept), diletakkan (is/was placed).
Never write disaku when you mean di saku.

Could -nya here mean “the” rather than “his/her”?

It could in other contexts, but in this sentence the presence of dia strongly cues a possessive reading. If you wanted a definite but non-possessive shirt pocket, you’d usually specify with itu: di saku kemeja itu.

Are pulpen, pena, and bolpoin all correct?

pulpen: very common general term for pen.
pena: also correct, sometimes feels a bit more formal or written.
bolpoin/bolpen: ballpoint pen (spelling bolpoin is standard). All are widely understood; usage varies by region and register.

Can I switch saya to aku here? Any register issues?

saya is polite/neutral; aku is informal/intimate. Both mean I.
Don’t mix saya and aku within the same context unless you intend a shift in tone or persona.
Colloquial Jakarta: gue/gua (very informal).

Why is it menyimpan (with ny) and merapikan (with r), not mensimpan?

It’s the meN- prefix assimilation rule:

Before roots starting with S, the S typically drops and meN- becomes meny-: simpan → menyimpan, sapu → menyapu.
Before roots starting with R, meN- surfaces as mer-: rapi → merapikan. This is standard Indonesian morphology.

Can I front objects in Indonesian, like “the tie I straightened”?

Yes. Object-fronting is common:

Dasinya saya rapikan. (Still active; the object is topicalized.) You can also use passive:
Pulpen disimpan di saku kemejanya, lalu dasinya dirapikan oleh saya/saya rapikan. In everyday speech, Dasinya saya rapikan sounds natural.