Learn Hardcore Indonesian: Aku melapor singkat lewat pesan, lalu mengirim materi terakhir. - I report briefly via a message, then send the latest material.

melapor = to report (intransitive), often with no object or followed by a person: melapor ke/kepada atasan. Your sentence has no object after “report,” so melapor fits.
melaporkan = to report something (transitive): you must have an object, e.g., Saya melaporkan insiden itu.
Bare lapor is the root; in casual speech you might see Aku lapor singkat, but standard writing prefers melapor.

Should it be melapor singkat or melapor secara singkat?

Both are acceptable. Indonesian often uses adjectives adverbially, so melapor singkat sounds natural in everyday use. melapor secara singkat is a bit more formal/explicit. Synonyms: singkat (brief), ringkas (concise).

What does lewat mean here, and are there alternatives?

lewat means “via/through/by means of.” Alternatives:

melalui (more formal)
via (loanword, neutral)
pakai (casual, “using”) So lewat pesan, melalui pesan, or via pesan all work.

Does pesan mean a text message specifically?

pesan is generic “message” (SMS, chat, platform DM, etc.). If you need to be specific, say:

pesan teks/SMS
pesan WhatsApp (WA)
pesan email (usually just email)

Why use lalu instead of dan, terus, or kemudian?

lalu = then/and then; neutral and common in writing to show sequence.
kemudian = then/later; slightly more formal or chronological.
terus = and then/kept on; very colloquial.
dan = and; just links items without a clear sense of “after.” Your sentence shows a sequence, so lalu is a good choice.

Can I drop the subject in the second clause? Is “lalu aku mengirim” better?

Omitting the repeated subject is natural when it’s the same as in the first clause. All are fine:

…, lalu mengirim materi terakhir. (elliptical, common)
…, lalu aku/saya mengirim materi terakhir. (explicit)
…, lalu kukirim materi terakhir. (literary/compact; ku- attaches to the verb as “I”)

Is mengirim different from mengirimkan or kirim?

mengirim [sesuatu] (kepada …) = standard “send.”
mengirimkan [sesuatu] (kepada …) = slightly more formal; often interchangeable with mengirim.
mengirimi [seseorang] [sesuatu] = emphasizes the recipient (e.g., mengirimi dia materi).
kirim/ngirim (without meN-) = casual speech. All would fit here; mengirim is the neutral default.

What exactly does materi mean? Is it natural?

materi usually means “content/materials” for learning, presentations, or documents/slides. It’s natural if you mean slides, attachments, or content to be reviewed. If you mean literal physical materials or files, you might also say dokumen, berkas, or file depending on context.

Does terakhir mean final or latest?

terakhir can mean either “final” (the last in a sequence) or “most recent,” and context disambiguates. If you need to be precise:

“final” = terakhir or final
“most recent/latest” = terbaru So you can say materi final or materi terbaru if needed.

Is materi terakhir the right word order? What about materi yang terakhir?

Yes, adjectives typically follow the noun: materi terakhir. materi yang terakhir is also correct but adds emphasis or clarification (like “the one that is last”), and is used when you want to specify or contrast.

Do I need the comma before lalu?

It’s optional. The comma before lalu often matches a natural pause and improves readability in writing: …, lalu …. Without the comma is also acceptable in short clauses.

Does melapor carry a hierarchical/politeness nuance?

Often yes. melapor suggests reporting to a superior or authority. If you just mean “give a quick update,” alternatives include:

memberi kabar singkat
mengabari singkat
mengirim laporan singkat (if you’re sending a short report document)

Any quick notes on the affixes here?

melapor = meN-
- lapor (report)
mengirim = meN-
- kirim (send); the k drops and meN- becomes meng-
terakhir = ter-
- akhir (the last) These are common, productive patterns worth recognizing.