Learn Hardcore Indonesian: Sesudah makan siang, saya mengirim dokumen ke kantor. - After lunch, I send the document to the office.

Yes. Both mean “after.” Setelah is slightly more common in neutral/formal writing; sesudah is equally correct. Informal options: sehabis/habis (and very casual abis).

Do I need a subject inside the “after”-clause? For example, can I say Sesudah saya makan siang?

Not required. Both are correct:

Sesudah makan siang, ... (After having lunch, …)
Sesudah saya makan siang, ... (After I have lunch, …) The version without a subject is very natural and concise.

Why is there a comma after the time phrase?

Standard Indonesian uses a comma after an initial adverbial phrase/clause. So Sesudah makan siang, ... is per the rule. If the time phrase goes at the end, you usually skip the comma: Saya mengirim ... sesudah makan siang.

Is there tense here? How do I show past vs future vs habitual?

Indonesian doesn’t mark tense on verbs; add time/aspect words:

Past/recent: tadi, barusan, kemarin (e.g., Tadi, saya mengirim ...)
Completed: sudah (e.g., Saya sudah mengirim ...)
Future: akan, nanti (e.g., Sesudah makan siang, saya akan mengirim ...)
Habitual: biasanya, setiap hari (e.g., Biasanya sesudah makan siang, saya mengirim ...)

Why mengirim and not just kirim?

The root is kirim. The active transitive form uses meN-: meN- + kirim → mengirim (the initial k drops after meng-). In casual speech, people often drop the prefix: Saya kirim dokumen ke kantor, but in neutral/formal style use mengirim.

What’s the difference between mengirim, mengirimkan, and mengirimi?

mengirim X ke/kepada Y = send X to Y (neutral, very common).
mengirimkan X ke/kepada Y = also “send X to Y”; the -kan form often sounds a bit more formal/emphatic on the thing being sent.
mengirimi Y X = send Y X (recipient as direct object), often without a preposition: Saya mengirimi dia dokumen.

Should I use ke, kepada, or untuk with “send”?

ke = to (a place): mengirim dokumen ke kantor.
kepada = to (a person/recipient), more formal: mengirim dokumen kepada manajer. Colloquially, ke with people also occurs.
untuk = for (for the benefit of): mengirim dokumen untuk kantor means “for the office,” not necessarily “to the office.”

Does ke kantor mean “to my office”? Do I need “the/a”?

Indonesian has no articles. ke kantor can mean “to the office,” usually understood from context as the speaker’s workplace. To be explicit:

specific: ke kantor itu / ke kantornya
my office: ke kantor saya
someone else’s: ke kantor Budi

Could I use di instead of ke?

Different meaning:

ke kantor = to the office (destination).
di kantor = at the office (location).
So mengirim dokumen di kantor means you did the sending while at the office, not that the document is going to the office.

Is makan siang a verb or a noun here?

It functions like either:

verbal idea: “after (I) eat lunch/after having lunch”
nominal idea: “after lunch”
Both readings are fine; Indonesian allows this kind of flexible verbal–nominal phrase.

How would I make it plural: “documents”?

Plural is optional. Options:

leave it unmarked: dokumen (context shows plurality)
reduplication: dokumen-dokumen
quantifiers/numbers: beberapa dokumen, dua dokumen If you want to count pages, use a classifier: dua lembar (two sheets).

Where else can the time phrase go?

All are natural:

Sesudah makan siang, saya mengirim dokumen ke kantor.
Saya mengirim dokumen ke kantor sesudah makan siang. Fronting the time phrase adds mild emphasis to the time.

How do I say this in the passive?

Two common options:

Short passive (actor expressed without di-): Sesudah makan siang, dokumen itu saya kirim ke kantor.
di- passive: Sesudah makan siang, dokumen itu dikirim ke kantor (oleh saya).
“Oleh saya” is usually omitted unless needed.

Can I drop the subject entirely?

If you drop it with a bare verb, it tends to sound imperative:

Sesudah makan siang, kirim dokumen ke kantor. = After lunch, send the document(s) to the office.
For a neutral statement, keep saya (or another subject).

I keep mixing up sudah and sesudah. Help?

sudah = already: Saya sudah mengirim dokumen.
sesudah = after: Sesudah makan siang, saya mengirim dokumen.
They look similar but mean different things.

Any register/word-choice tips for everyday vs formal speech?

Formal/neutral: Setelah/Sesudah makan siang, saya mengirim/mengirimkan dokumen ke/kepada kantor/manajer.
Casual: Habis/Abis makan siang, aku/gue kirim dokumen ke kantor.
Match pronouns and affixes to the setting.