Learn Hardcore Indonesian: Dia belum yakin memilih jadwal, lagipula pilihannya banyak. - She/He isn’t sure about choosing a schedule yet; besides, there are many options.

"Belum" means "not yet" and implies the situation could change. "Dia belum yakin ..." suggests the person may become sure later.
"Tidak" is a plain negation. "Dia tidak yakin ..." means "he/she is not sure," with no implication of change.

Is "yakin" an adjective or a verb? What can follow it?

"Yakin" is an adjective meaning "sure/confident," and adjectives can function as predicates in Indonesian (no "to be" needed): "Dia yakin."
Common complements:
- yakin (bahwa) + clause: "Dia yakin (bahwa) rencana itu berhasil."
- yakin + verb phrase (colloquial but common): "Dia yakin memilih A" = confident about choosing A.
- yakin untuk + verb: widely used in speech/writing; "Dia yakin untuk memilih A." ("untuk" is optional stylistically).
- yakin akan + noun: "Dia yakin akan kemenangannya." Note: this "akan" does not go before a verb.

Is "Dia belum yakin memilih jadwal" natural, or should I say it differently?

It’s natural, especially in conversation. Alternatives:

"Dia belum yakin untuk memilih jadwal."
"Dia belum yakin memilih jadwal yang mana."
"Dia belum yakin dengan jadwal yang akan dipilih."
"Dia belum yakin menentukan jadwal." (slightly more formal)

What does "lagipula" do here? How formal is it? Any synonyms?

"Lagipula" adds an extra, supportive reason: "besides/anyway/after all."
Register: informal–neutral; fine in speech and casual writing. In formal writing prefer "selain itu" or "di samping itu."
Not the same as "karena" ("because"), which directly marks a cause.

Do I need a comma before "lagipula"? Could it start a new sentence?

Yes, it’s common to write a comma before it: "... , lagipula ...". You can also start a new sentence: "Lagipula, pilihannya banyak."

In "pilihannya banyak," what does the suffix -nya mean?

"-nya" can mark possession (his/her/its) or definiteness ("the"). Here it’s definiteness: "the choices (in question) are many," not necessarily "his/her choices."

Could I say "Ada banyak pilihan" instead? How does it differ from "pilihannya banyak"?

"Ada banyak pilihan" = "There are many choices" (neutral existence).
"Pilihannya banyak" = "The choices (we’ve been talking about) are many" (focus on that known set).

Is "banyak pilihannya" also correct? What’s the nuance vs "pilihannya banyak"?

Yes. "Banyak pilihannya" front-loads the emphasis on "many" and is common in speech; "Pilihannya banyak" is a bit more neutral. Meaning is the same.

Does "pilihannya banyak" mean "she has many choices"?

Not necessarily. It describes the number of available options, not ownership. To say she has many options, use "Dia punya banyak pilihan" (or formal "Dia memiliki banyak pilihan").

Why isn’t there "yang mana" after "jadwal"? How do I explicitly say "which schedule"?

It’s implied. Indonesian often omits predictable parts. To be explicit, say: "Dia belum yakin memilih jadwal yang mana."

Can I drop the subject pronoun "Dia" here?

Yes, if context makes the subject clear. "Belum yakin memilih jadwal, lagipula pilihannya banyak" is perfectly natural in conversation.

What’s the difference between "memilih", "pilih", and "milih"?

"memilih": standard active verb "to choose" (use in most writing).
"pilih": base form; common in imperatives: "Pilih satu!"
"milih": colloquial spoken form (dropping meN-). Also, "memilih-milih" = "to be picky."

Should I use "jadwal" or "waktu" here?

"jadwal": a schedule/slot from a plan or timetable (choosing among listed options).
"waktu": time in general. Use "memilih waktu" if you’re just picking a time, "memilih jadwal" if selecting among scheduled slots.

Can I add "akan" to mark the future: "Dia belum yakin akan memilih jadwal"?

Be careful:

With verbs, "akan" marks future: "not yet sure he will choose a schedule (at all)," slightly shifting the meaning.
With nouns, "yakin akan + noun" means "sure about": e.g., "Dia yakin akan keputusannya." You cannot use this prepositional "akan" directly before a verb.

Can "yakin" be followed by "bahwa"?

Yes, when followed by a full clause: "Dia yakin (bahwa) rencana itu tepat." In this context you could recast: "Dia belum yakin bahwa jadwal A yang terbaik," but you wouldn’t put "bahwa" right before a bare verb phrase like "memilih jadwal."

How would a more formal version of the whole sentence look?

"Ia belum yakin menentukan jadwal; selain itu, pilihannya banyak."
"Ia belum yakin menetapkan jadwal. Di samping itu, terdapat banyak pilihan."