Learn Hardcore Indonesian: Dia memesan taksi lewat aplikasi, lalu memberi tahu saya. - He/She orders a taxi via the app, then informs me.

Neutral/informal: lewat aplikasi
More formal: melalui aplikasi
Also common: pakai/menggunakan aplikasi
Focus on the place rather than the means: di aplikasi (fine in everyday speech to mean “in the app,” but it emphasizes location more than method)

All of these are acceptable; choose based on formality and focus.

Why is it memesan? Could I use memanggil, menyewa, or just pesan?

Memesan = to order/reserve (appropriate for ordering a taxi by phone/app).
Memanggil = to call/hail (waving one down, calling a dispatcher).
Menyewa = to hire/rent (often for a longer period, e.g., renting a car with a driver).
Base form pesan appears in imperatives or casual writing: Pesan taksi! but in a full sentence with a subject, memesan is standard.

Form note: memesan = meN- + pesan; the initial p of pesan drops after the prefix, so you get meN + (p)esan → memesan.

Does Dia mean “he” or “she”? How is it different from ia or beliau?

Dia is gender-neutral: he/she.
Ia is also gender-neutral but more literary/formal and mostly used as a subject in writing.
Beliau is a respectful/honorific “he/she” for elders or people of higher status.

There’s no past tense marking. How do we know it’s past, and how can I make it explicitly past?

Indonesian verbs don’t inflect for tense. Sequence and context imply time. To make it explicit:

Completion: Dia sudah memesan…
Recent past: Baru saja dia memesan… / Tadi dia memesan…
Yesterday: Kemarin dia memesan…
Formal: Dia telah memesan… You can also say: Setelah memesan taksi lewat aplikasi, dia memberi tahu saya.

Is it spelled taksi or taxi?

Standard Indonesian spelling is taksi. You’ll still see taxi in brand names or signage, but in writing, use taksi.

What’s the difference between memberi tahu, memberitahu, and memberitahukan? Which fits best here?

All mean “to inform/tell,” but patterns and register differ:

Memberi tahu [person] (tentang [thing]) / bahwa…
- Everyday/neutral. Example: Dia memberi tahu saya.
Memberitahu [person] (tentang [thing]) / bahwa…
- Slightly more formal; also common. Example: Dia memberitahu saya.
Memberitahukan [thing] kepada [person]
- More formal and prefers the pattern with kepada. Example: Dia memberitahukan rencananya kepada saya.

For your sentence, memberi tahu saya or memberitahu saya are both natural.

Why is it memberi tahu saya and not memberi tahu kepada saya?

With memberi tahu, the person informed is a direct object, so use memberi tahu saya. If you want to use kepada, switch to the -kan form:

Correct: Dia memberitahukan hal itu kepada saya.
Also correct (no -kan): Dia memberitahu saya hal itu.
Avoid: memberi tahu kepada saya (nonstandard mix).

Can I say kasih tahu instead of memberi tahu? What about mengabari?

Yes:

Kasih tahu (or spoken ngasih tahu) is very common and informal: Dia kasih tahu saya.
Mengabari (spoken ngabarin) is also common: Dia mengabari saya. Use these with friends/in casual contexts; in formal writing, prefer memberi tahu/memberitahu.

Is the comma before lalu necessary?

It’s optional. A comma helps separate two clauses for clarity:

With comma: …, lalu …
Without comma: … lalu … Both are acceptable.

Can I repeat the subject in the second clause: Dia memesan…, lalu dia memberi tahu saya?

Yes, that’s fine. Indonesian often drops a repeated subject when it’s clear, but repeating dia is also correct, especially for emphasis or clarity.

How do I say this in the passive voice?

Two natural options:

Taksi dipesan lewat aplikasi, lalu saya diberi tahu.
Setelah taksi dipesan lewat aplikasi, saya diberi tahu. These focus on the action/result rather than the doer.

How do I add what the person told me?

Use either a clause or a noun phrase:

Clause with “that”: Dia memberi tahu saya bahwa ia sudah berangkat. (Colloquial: … kalau dia sudah berangkat.)
Noun phrase with “about”: Dia memberi tahu saya tentang perubahan jadwal.

Does lewat aplikasi mean “via an app” or “via the app”? How do I specify?

Indonesian lacks articles, so lewat aplikasi can be either. To be specific:

lewat aplikasi itu = via the app
lewat aplikasinya = via the app (often sounds definite; can also mean “his/her app” from context)
lewat aplikasi Grab/Gojek = via [brand] app

Does pesan also mean “message”? How do I avoid confusion with memesan?

Yes, pesan can be:

A noun “order/reservation” or the verb base “to order”
A noun “message” Context and affixes disambiguate:
Ordering: Saya memesan taksi.
Messaging: Saya mengirim pesan. / Saya menerima pesan. In your sentence, memesan clearly means “to order/reserve.”