Learn Hardcore Indonesian: Kami berkemas pelan-pelan malam sebelumnya sehingga tidak ada perlengkapan yang tertinggal. - We packed slowly the night before so that no equipment was left behind.

berkemas = to pack up / to get packed (intransitive, no direct object)
- Kami berkemas. = We packed up.
mengemas (sesuatu) = to pack something (transitive, takes an object)
- Kami mengemas barang-barang kami. = We packed our things.

In the sentence Kami berkemas pelan-pelan..., the focus is on the activity of packing, not on what is packed, so berkemas is natural.
If you want to emphasize the items, you can say:

Kami mengemas perlengkapan kami pelan-pelan malam sebelumnya.

Why is pelan-pelan repeated? What’s the difference between pelan and pelan-pelan?

This is Indonesian reduplication, often used to:

Soften the meaning
Make it sound more natural or casual
Emphasize a gradual or gentle action

pelan = slow
pelan-pelan = slowly / gently / take your time

Both are understandable, but:

Kami berkemas pelan. sounds a bit more neutral or even slightly stiff.
Kami berkemas pelan-pelan. sounds more natural in everyday speech; it suggests they deliberately took their time and did it carefully.

A close synonym is perlahan-lahan:

Kami berkemas perlahan-lahan. (a bit more formal/literary)

Can the position of pelan-pelan change? Is Kami pelan-pelan berkemas also correct?

Yes, adverbs like pelan-pelan are quite flexible.

All of these are grammatical, with slightly different rhythms/emphasis:

Kami berkemas pelan-pelan malam sebelumnya.
(most natural; standard word order: subject–verb–manner–time)
Kami pelan-pelan berkemas malam sebelumnya.
(emphasizes the manner a bit more)
Pelan-pelan kami berkemas malam sebelumnya.
(puts strong emphasis on slowly, somewhat literary or stylistic)

In everyday speech and writing, the original Kami berkemas pelan-pelan malam sebelumnya is the most neutral and common.

Indonesian verbs don’t change for past tense, so how do we know this is in the past?

Indonesian usually shows time through time expressions, not verb changes.

In this sentence, malam sebelumnya (the previous night / the night before) tells you it happened in the past. There is no need to change berkemas.

So:

Kami berkemas pelan-pelan malam sebelumnya.
= We packed slowly the night before.

Compare:

Kami berkemas pelan-pelan setiap malam.
= We pack slowly every night. (present/habitual from setiap malam)
Kami akan berkemas pelan-pelan besok malam.
= We will pack slowly tomorrow night. (akan
- besok malam give future meaning)

What exactly does malam sebelumnya mean, and how is it different from tadi malam, semalam, or malam itu?

All refer to a night in the past, but with different nuances:

malam sebelumnya = the previous night / the night before (relative to some reference point in the story)
- Often used in narratives: referring to the night before another event.
tadi malam = last night (the night that just passed, from the speaker’s point of view now)
semalam = last night / all night
- Depending on context, can mean last night or throughout the night.
malam itu = that night
- Refers to a specific night already known in the story or conversation.

In this sentence, malam sebelumnya likely means the night before some main event (e.g. before a trip, before a competition, etc.).

Why is there no pada before malam sebelumnya? Is pada malam sebelumnya also correct?

Yes, pada malam sebelumnya is also grammatically correct.

pada is a preposition similar to on/at, often used with time expressions.
In everyday Indonesian, pada is frequently dropped when the meaning is clear.

So:

Kami berkemas pelan-pelan malam sebelumnya.
Kami berkemas pelan-pelan pada malam sebelumnya.

They mean the same thing; the first is more natural in informal to neutral style.

What does sehingga do here? How is it different from jadi or agar/supaya?

sehingga introduces a result clause, similar to so that / as a result.

...sehingga tidak ada perlengkapan yang tertinggal.
= ...so that / with the result that no equipment was left behind.

Differences:

sehingga = result (often more neutral or formal)
- Dia terlambat bangun, sehingga dia ketinggalan bus.
  He woke up late, so (as a result) he missed the bus.
jadi = so / so then (more conversational)
- Dia terlambat bangun, jadi dia ketinggalan bus.
agar / supaya = so that / in order that (indicates purpose, intention)
- Kami berkemas pelan-pelan agar tidak ada perlengkapan yang tertinggal.
  We packed slowly so that no equipment would be left behind.

In your sentence, sehingga emphasizes the result of what actually happened, not just the intention.

What does perlengkapan mean exactly, and how is it different from peralatan or just barang-barang?

Roughly:

perlengkapan = supplies / gear / equipment (things you need to support an activity)
- perlengkapan camping = camping gear
- perlengkapan sekolah = school supplies
peralatan = tools / equipment (more about tools, instruments, devices)
- peralatan dapur = kitchen tools
- peralatan medis = medical equipment
barang-barang = things / stuff (very general)

In this sentence:

perlengkapan suggests specific items needed for something (a trip, event, activity), not just random stuff.

What does yang tertinggal mean structurally? How does the ter- prefix work here?

yang tertinggal literally breaks down as:

yang = that / which / who (introduces a relative clause)
tertinggal = left behind / remaining

So perlengkapan yang tertinggal = equipment that was left behind.

About ter-:

ter- often marks:
- a state/result: tertutup = closed; terbuka = open
- something accidental/unintentional: tersesat = got lost (not on purpose)
- a natural superlative: terbaik = best; terbesar = biggest

Here tertinggal suggests a state of ending up left behind, often unintentionally.

Compare:

Kami meninggalkan beberapa perlengkapan.
We left some equipment (we are the agent, more active).
Ada beberapa perlengkapan yang tertinggal.
There was some equipment that ended up left behind (focus on the items and the result).

Why is it tidak ada perlengkapan and not tidak perlengkapan?

In Indonesian, to say “there is no X / there are no X”, you use:

tidak ada + noun

So:

Tidak ada perlengkapan. = There is no equipment.
Tidak ada orang. = There is no one.
Tidak ada masalah. = There is no problem.

tidak perlengkapan is ungrammatical in this meaning.

You may also hear tak ada or nggak ada in speech:

Tak ada perlengkapan yang tertinggal. (more literary/formal or poetic)
Nggak ada perlengkapan yang tertinggal. (informal spoken Indonesian)

Could the sentence be reordered, for example starting with Malam sebelumnya?

Yes, time expressions can be placed at the beginning for emphasis:

Malam sebelumnya, kami berkemas pelan-pelan sehingga tidak ada perlengkapan yang tertinggal.

This is fully correct and quite natural, especially in narratives, where you often start with a time or place phrase. The meaning does not change; you just highlight when first.

Why is kami used here instead of kita? What’s the difference?

Both mean we, but:

kami = we (excluding the listener)
- “we but not you”
kita = we (including the listener)
- “we including you”

In a story, if the speaker is talking about their group and the listener was not part of that group, kami is correct:

Kami berkemas pelan-pelan malam sebelumnya...
We (my group, not including you) packed slowly the night before...

If the listener was part of the group that packed, then kita would be used:

Kita berkemas pelan-pelan malam sebelumnya...
We (you and I and the others) packed slowly the night before...