Learn Hardcore Indonesian: Di jalan menuju air terjun, saya melihat jejak kecil di tanah basah di antara batu besar. - On the path to the waterfall, I see small tracks on the wet ground between large rocks.

di jalan menuju air terjun = on the road that leads to the waterfall or on the road heading to the waterfall.
If you say di jalan ke air terjun, it still means on the road to the waterfall, but menuju emphasizes the direction/orientation toward the waterfall a bit more strongly.

So:

menuju often carries a sense of intentional movement or direction.
ke is more neutral and is used for many kinds of to (to school, to the office, to Jakarta, etc.).

Both are correct here; menuju just sounds a bit more descriptive and vivid.

Why is it air terjun for waterfall and not terjun air?

In Indonesian, the usual pattern is head noun + modifier, not the other way around like English.

air = water
terjun (as a noun here) = fall (as in a drop/fall), or as a verb to dive/jump down

So air terjun literally means falling water or water that falls, which corresponds to the English word waterfall.

You cannot say terjun air to mean waterfall; that would sound like a verb phrase (to dive into water, depending on context) rather than a noun.

Other similar patterns:

kaca mata → kacamata = glasses (eye glass(es))
rumah sakit = hospital (sick house)

So remember: Indonesian generally puts the main noun first (air) and then what describes it (terjun).

Why does the sentence start with Di jalan menuju air terjun and use a comma? Is that necessary?

Starting with Di jalan menuju air terjun sets the scene first: On the road to the waterfall, I saw…. This is very natural in Indonesian.

About word order:

Current version: Di jalan menuju air terjun, saya melihat jejak kecil …
Also correct: Saya melihat jejak kecil … di jalan menuju air terjun.

Both are grammatical. The difference is mostly about style and emphasis:

Fronting Di jalan menuju air terjun emphasizes the location and gives a storytelling feel.
Putting it at the end is more neutral.

About the comma:

You will see both with and without a comma:
- Di jalan menuju air terjun, saya melihat…
- Di jalan menuju air terjun saya melihat…
In practice, the comma here is stylistic, to mark the pause. It is helpful but not strictly required by Indonesian grammar rules.

There are three di in this sentence. Are they all the same word, and how do I know when di is a preposition or the passive prefix?

In this sentence, all the di are prepositions of place:

di jalan = on the road
di tanah basah = on the wet ground
di antara batu besar = among / between the big rock(s)

As a preposition, di means in / on / at, depending on context.

Indonesian also has di- as a prefix marking passive verbs, for example:

melihat = to see
dilihat = to be seen

How to tell the difference:

Preposition di:
- Written separately from the next word: di jalan, di rumah, di sekolah.
- The word after it is usually a noun or a noun phrase.
Passive prefix di-:
- Written attached to a verb with no space: dilihat, dibaca, ditulis.
- The whole thing is a verb.

In your sentence, because all the di are followed by spaces and then place nouns (jalan, tanah, antara), they are clearly the preposition di meaning at / in / on.

How do I know if jejak kecil is singular or plural? How can I say one small footprint vs some small footprints?

Indonesian normally does not mark singular vs plural on the noun. So:

jejak kecil can mean:
- a small footprint
- small footprints
  depending on context.

If you want to be explicit:

satu jejak kecil = one small footprint
beberapa jejak kecil = several small footprints
banyak jejak kecil = many small footprints

You can also use reduplication to show plurality:

jejak-jejak kecil = small footprints (clearly plural)

However, native speakers very often just say jejak kecil and let the context tell you whether it is one or more.

Does jejak always mean footprint, or can it be other kinds of marks?

Jejak is more general than just footprint. It can mean:

footprint / paw print / hoof mark (marks left by feet)
tracks / trail (marks or signs left behind by movement)
trace / sign (more abstract, like traces of someone’s presence)

For specifically footprint(s), you might say:

jejak kaki = footprint(s) (literally foot traces)

So:

jejak kecil = small traces / small tracks / small footprints (depending on the story)
jejak kaki kecil = clearly small footprints

In a nature context with ground and rocks, jejak kecil can naturally be understood as small animal or human footprints.

Why do the adjectives kecil, basah, and besar come after the nouns in Indonesian?

In Indonesian, the normal pattern is:

Noun + Adjective

So in your sentence:

jejak kecil = small footprint(s) (footprint small)
tanah basah = wet ground (ground wet)
batu besar = big rock(s) (rock big)

Adjectives like kecil (small), basah (wet), besar (big) usually follow the noun they describe, unlike English where adjectives usually come before the noun.

They also do not change form for singular/plural or gender:

batu besar can be a big rock or big rocks.
batu besar itu can be that big rock or those big rocks, depending on context.

Why is it batu besar and not batu-batu besar or batu besar-besar if we are “among big rocks”? How do you express plural here?

Indonesian normally leaves plurality to context, so batu besar can already mean:

a big rock
big rocks

Because di antara (among / between) often implies more than one object, di antara batu besar is naturally understood as among big rocks, even without a plural marker.

If you want to highlight plurality more clearly, you have options:

Reduplicate the noun:
- di antara batu-batu besar = among the big rocks
  (clearly more than one rock)
Reduplicate the noun + adjective:
- di antara batu besar-besar
  Also means among big rocks, often with a nuance that each rock is big.
Use a quantifier:
- di antara beberapa batu besar = among several big rocks
- di antara banyak batu besar = among many big rocks

In normal storytelling, di antara batu besar is perfectly natural and will usually be understood as plural.

What is the difference between di antara and antara by itself?

Both involve the idea of between / among, but they are used slightly differently in practice.

antara by itself:
- Often used when you mention two things explicitly:
- antara batu besar dan pohon = between the big rock and the tree
- antara Jakarta dan Bandung = between Jakarta and Bandung
di antara:
- Literally in between / among.
- Common when the subject is inside or among a group of things:
- di antara batu besar = among the big rock(s)
- di antara teman-teman saya = among my friends

In your sentence, di antara batu besar is correct and natural, because you are located in the midst of rock(s). Saying just antara batu besar here would sound incomplete or odd.

Could I use aku instead of saya here? What is the difference?

Yes, grammatically you can use aku instead of saya:

saya melihat jejak kecil…
aku melihat jejak kecil…

Both mean I saw small footprints….

The difference is formality and relationship:

saya
- Polite, neutral, and safe in almost all situations.
- Used in formal speech, with strangers, with people older than you, at work, in writing, etc.
aku
- Informal and more intimate.
- Used with friends, family, people of similar age, and often in songs, poems, and stories.

In a neutral narrative sentence like this, you could choose either:

In a formal story or essay: saya is more standard.
In a personal diary, a casual story, or children’s story: aku is very common.

For learners, using saya is usually the safest default until you are comfortable with social nuances.