Learn Hardcore Indonesian: Saat belajar mandiri, saya menyimpan semua tugas di folder khusus lalu menyortirnya berdasarkan tanggal. - When studying independently, I store all assignments in a special folder and then sort them by date.

ketika → ketika belajar mandiri
Very natural, almost the same meaning as saat.
waktu → waktu belajar mandiri
Also possible, a bit more informal in many contexts.

In this sentence, saat, ketika, and waktu all work and mean when here.
saat and ketika often feel a bit more neutral or slightly formal than waktu in writing.

Why is there no saya after saat? Could I say Saat saya belajar mandiri?

The full form could indeed be:

Saat saya belajar mandiri, saya menyimpan semua tugas …

Indonesian often drops repeated subjects when the meaning is clear. So:

Saat belajar mandiri, saya …

is understood as When I study independently, I … even though saya is not repeated in the saat-clause.

Both are correct:

With explicit subject: Saat saya belajar mandiri, saya menyimpan …
With omitted subject: Saat belajar mandiri, saya menyimpan …

Omitting the subject in the first clause sounds natural and not incomplete in Indonesian.

What is the nuance of belajar mandiri? How is it different from belajar sendiri?

Both are related but have different nuances:

belajar mandiri
Focuses on independent, self-directed learning. It sounds a bit more formal or academic, like independent study or self-directed learning.
belajar sendiri
Literally to study alone/by oneself.
This can mean:
- physically alone (no one with you), or
- not being helped by others.

So:

Saya belajar mandiri.
Emphasis on independence as a method/approach.
Saya belajar sendiri.
Emphasis more on not with others (not in a group, not with a tutor), though it can overlap with mandiri.

In your sentence, belajar mandiri is a good phrase if you mean self-study rather than simply studying alone in a room.

Why is it menyimpan, and what is the base word? Is there a rule here?

The base word is simpan (to keep / to store).
The verb menyimpan is formed with the prefix meN-:

meN-
- simpan → menyimpan

Rule: when meN- attaches to a verb starting with s, s often drops and meN- becomes meny-:

meN- + sapu → menyapu (to sweep)
meN- + simpan → menyimpan (to store/keep)

So menyimpan is an active transitive verb (to store/keep something).
It fits the pattern [subject] + menyimpan + [object]:

Saya menyimpan semua tugas …
I store/keep all the assignments …

What is the difference between menyimpan and menaruh? Could I say menaruh semua tugas di folder khusus?

Both can work, but the nuance is different:

menyimpan
to store/keep in an organized, more permanent way. Good for:
- files
- documents
- money
- data
menaruh
to put/place (somewhere). Focus is more on physically placing, not on long-term storage or organization.

In the context of managing assignments (especially digital files or organized documents), menyimpan is more appropriate because you are storing them in a specific folder for future use.

menaruh semua tugas di folder khusus is understandable, but it sounds more like just putting them there rather than properly saving/filing them.

Why is it semua tugas and not something like a plural form tugases? How does plural work here?

Indonesian generally does not add an ending to make plurals like English -s.

Plurality is shown by:

Context, or
Words like semua (all), banyak (many), beberapa (several),
Sometimes reduplication: tugas-tugas (tasks/assignments).

In this sentence:

tugas by itself can be task or tasks.
semua tugas clearly means all tasks / all assignments.

So you do not need to change tugas itself; semua already marks it as plural.

Why is the preposition di used in di folder khusus instead of ke?

Indonesian distinguishes:

di = at/in/on (location, where something is)
ke = to (movement/direction, where something goes)

In di folder khusus, the focus is on the location where the tasks are stored:

menyimpan … di folder khusus
store … in a special folder

If you said ke folder khusus, it would sound more like the act of moving something to that folder, often more physical/directional:

memindahkan file ke folder khusus
move the file to a special folder

For the idea of keeping/storing in that folder as their place, di is the natural choice.

Why is it folder khusus and not khusus folder? What is the adjective order in Indonesian?

In Indonesian, adjectives normally come after the noun.

Pattern: [noun] + [adjective]

Examples:

folder khusus = special folder
buku baru = new book
tugas penting = important assignment

So folder khusus is correct and natural.

Putting the adjective before the noun (khusus folder) is not standard and would sound wrong to native speakers in this context.

What does lalu do here? Could I use kemudian or just dan instead?

In the sentence:

… di folder khusus lalu menyortirnya berdasarkan tanggal.

lalu is a connector meaning then / and then / after that. It shows sequence:

store all the assignments in a special folder
then sort them by date

Alternatives:

kemudian
Very similar meaning, often slightly more formal or neutral.
… di folder khusus kemudian menyortirnya … is fine.
dan
Means simply and, without emphasizing sequence as strongly.
… di folder khusus dan menyortirnya … is grammatically possible but weaker in showing first this, then that.

In writing and speech, lalu and kemudian are both very common for then.

What does the -nya in menyortirnya refer to, and why is it used?

The base verb is menyortir (to sort).
The -nya is an object pronoun here, roughly them/it, referring back to semua tugas.

So:

menyortirnya ≈ sort them (the assignments)

If we break it down:

menyortir = to sort
-nya = it / them / that

Without -nya, the sentence would feel incomplete or at least less clear, because you would lose the direct object that was just mentioned:

Saya menyimpan semua tugas … lalu menyortir berdasarkan tanggal.
This is still understandable, but native speakers strongly prefer menyortirnya to tie it back to semua tugas.

Other ways to refer back:

menyortir semua tugas itu berdasarkan tanggal
menyortir tugas-tugas tersebut berdasarkan tanggal

But menyortirnya is compact and very natural.

Why is the verb menyortir spelled that way? What is the base word?

The base word is sortir (to sort), a loanword (via Dutch).
With the active prefix meN-, it becomes menyortir:

meN-
- sortir → menyortir

Again, this follows the same pattern as with simpan:

Initial s is dropped
meN- becomes meny-
The rest of the word stays: ortir

So the active verb is menyortir (to sort something).

What does berdasarkan tanggal mean exactly, and how is berdasarkan used?

berdasarkan means based on or according to.
It comes from dasar (base, foundation) with prefixes.

In berdasarkan tanggal:

berdasarkan = based on, according to
tanggal = date (calendar date)

So berdasarkan tanggal = by date / based on the date.

Usage pattern: berdasarkan + noun

Examples:

berdasarkan aturan = based on the rules
berdasarkan data = based on the data
berdasarkan pengalaman = based on experience

In your sentence, menyortirnya berdasarkan tanggal = sort them by date.

Why is there a comma after Saat belajar mandiri? Is that required?

The structure is:

Dependent clause first: Saat belajar mandiri, …
Main clause after: … saya menyimpan semua tugas …

In Indonesian, when a dependent clause (time clause, condition, etc.) comes before the main clause, it is standard to put a comma:

Saat belajar mandiri, saya menyimpan …
Kalau hujan, saya tidak pergi.

If you reverse the order, the comma is usually not needed:

Saya menyimpan semua tugas di folder khusus saat belajar mandiri.

So in your sentence, the comma after Saat belajar mandiri is correct and natural.