Learn Hardcore Indonesian: Guru datang lebih awal untuk memastikan jadwal rapat sudah jelas. - The teacher comes earlier to make sure the meeting schedule is already clear.

Context decides. In this sentence, Guru datang... most naturally means The teacher came..., referring to some specific teacher the speakers know about. But grammatically, Indonesian doesn’t force you to choose singular/plural or definite/indefinite unless you add words like:

seorang guru = a (single) teacher
para guru = the teachers (group, respectful)
guru-guru = teachers (plural, general)

So guru alone is neutral; English has to pick the teacher because English requires an article and number.

Why is there no word for the in Guru datang...?

Indonesian does not use articles like a/an/the. Nouns stand alone:

guru = a teacher / the teacher / teachers / the teachers
buku = a book / the book / books

Definiteness is usually clear from context or is made explicit with other words (like ini for this, itu for that):

guru itu = that teacher / the teacher
buku ini = this book / the book

How do we know the sentence is in the past (came early) when datang has no tense?

Indonesian verbs do not change form for tense. datang simply means come. Past, present, or future are understood from context or time expressions:

Guru datang lebih awal…
In a narrative about something that already happened, this is understood as The teacher came earlier…

To make time clearer, speakers can add adverbs:

Guru tadi datang lebih awal = The teacher came earlier (just now/earlier today)
Guru akan datang lebih awal = The teacher will come earlier

So the same form datang is used, and time is indicated by other words or context.

Why datang and not tiba? What is the difference?

Both datang and tiba can translate as to arrive, but:

datang is more general and more common in everyday speech.
tiba sounds a bit more formal/literary and is often used with places or times:
- tiba di Jakarta = arrive in Jakarta
- tiba pukul 8 = arrive at 8 o’clock

In your sentence, Guru datang lebih awal... is the most natural everyday way to say The teacher came earlier....
Guru tiba lebih awal... is grammatically fine, but sounds a little more formal or report-like.

Is lebih awal literally more early? Why not just say awal?

Yes, lebih awal literally is more early, but in practice it means earlier.

awal by itself is an adjective meaning early:
- Dia orang yang datang awal. = He is someone who comes early.
lebih is a comparative marker (more/-er):
- lebih besar = bigger / larger
- lebih penting = more important

So:

lebih awal = earlier / more early

Saying Guru datang awal is possible, but it would more likely be interpreted as The teacher comes early (as a regular habit or absolute description), not came earlier than usual / than others.
lebih awal clearly expresses a comparison to some expectation or reference time.

What is the difference between lebih awal and lebih cepat?

Both could be translated as earlier, but they focus on different ideas:

awal = early (in terms of time, schedule)
cepat = fast/quick (in terms of speed)

So:

Guru datang lebih awal
Focus: the time of arrival is earlier than usual / than planned.
Guru datang lebih cepat
Focus: the teacher came faster (maybe traveled more quickly, finished something faster). It can sometimes be understood as earlier, but the nuance is more about speed than about schedule.

For earlier than usual or earlier than planned in a scheduling sense, lebih awal is the natural choice.

Can I change the word order and say Guru lebih awal datang?

That is not natural here. The normal order is:

[Subject] [Verb] [Adverb]
Guru datang lebih awal

Common patterns:

Dia datang terlambat. = He came late.
Kami berangkat pagi-pagi. = We left early in the morning.

Putting the adverb before the verb (lebih awal datang) usually sounds odd or marked. Stick with datang lebih awal.

What exactly does untuk do in untuk memastikan?

untuk here introduces a purpose, similar to to or in order to in English.

Structure:

untuk + verb = to (do something) / in order to (do something)

Examples:

Saya belajar keras untuk lulus ujian.
I study hard to pass the exam.

In ...datang lebih awal untuk memastikan..., untuk introduces the reason/purpose for coming early:

came early to make sure...
came early in order to make sure...

So:

datang lebih awal = came earlier
untuk memastikan... = to make sure (that)...

What is the structure and meaning of memastikan?

memastikan comes from:

base adjective: pasti = certain, sure
prefix + suffix: meN-
- -kan
result: memastikan = to make certain, to ensure, to make sure

Pattern: meN- + [adjective/noun] + -kan often means to cause (something) to be X or to make X happen.

So memastikan literally means to make sure / to ensure.

Usage:

memastikan jadwal rapat sudah jelas
= ensure (that) the meeting schedule is already clear
Dia memastikan pintu terkunci.
He made sure the door was locked.

You can also use bahwa to introduce a full clause:

Dia memastikan bahwa semua orang tahu.
He made sure that everyone knew.

Why is it jadwal rapat and not something like rapat jadwal?

In Indonesian, the typical noun–noun structure is:

Head noun + modifying noun

So:

jadwal rapat
head: jadwal (schedule)
modifier: rapat (meeting)
= meeting schedule / schedule of the meeting

Other examples:

rumah sakit = hospital (literally: house [of] sickness)
kartu mahasiswa = student card

If you said rapat jadwal, it would sound like a schedule meeting (a meeting about the schedule), which is not what you want here. The written form jadwal rapat is the standard noun phrase schedule of the meeting.

What does sudah add to jadwal rapat sudah jelas? How is that different from just jadwal rapat jelas?

sudah means already and often marks that a state has been achieved or completed.

jadwal rapat jelas = the meeting schedule is clear
(stating a fact, without highlighting any change)
jadwal rapat sudah jelas = the meeting schedule is already clear / is now clear
(implies a change: previously unclear or being prepared, now it has become clear)

So sudah adds:

a sense of completion or new state reached
sometimes a contrast with what was expected (e.g., you might expect it to still be unclear, but actually it is already clear)

Is sudah here more like already or like a past tense marker?

In this sentence, sudah is better thought of as already rather than a tense marker.

It does not directly mark past tense like -ed in English.
It shows that a state has been achieved or an action has been completed.

Examples:

Saya sudah makan.
I have (already) eaten.
Tugasnya sudah selesai.
His homework is already finished.

In jadwal rapat sudah jelas, the idea is the schedule is already clear / has become clear, not just was clear sometime in the past. Time is still inferred from context.

Could we replace sudah with telah here? What is the difference?

Grammatically, yes:

jadwal rapat telah jelas

However:

telah is more formal, and more common in writing (news articles, official reports, formal announcements).
sudah is neutral and widely used in speech and writing.

In everyday spoken Indonesian, sudah is strongly preferred.
telah would sound more formal, or like written/report language:

Rencana kegiatan telah disetujui.
The activity plan has been approved. (formal)

Is jelas an adjective here (clear) or a verb (to clarify)?

In jadwal rapat sudah jelas, jelas functions as an adjective: clear.

Structure:

[Subject] [aspect marker] [adjective]
jadwal rapat = subject
sudah = aspect marker (already)
jelas = adjective

So the whole clause means:

the meeting schedule is already clear / is now clear

There is also a verb menjelaskan (from the same root) which means to explain / to clarify, but jelas itself here is an adjective, not a verb.

What is the formality level and tone of this sentence? Is it polite?

The sentence is in standard, neutral Indonesian:

Vocabulary (guru, datang, lebih awal, memastikan, jadwal rapat, sudah jelas) is common and not slang.
No special honorifics, but it is fully polite in most contexts.

It would fit well:

in an email to colleagues,
in spoken conversation in an office,
in a written report (possibly with small tweaks for very formal style).

To make it slightly more formal, someone might add details or use telah instead of sudah, but as-is it’s perfectly acceptable and polite in most everyday and semi-formal situations.