Learn Hardcore Indonesian: Jalan kecil itu berbahaya saat hujan deras. - That small road is dangerous when it rains heavily.

Noun + adjective + itu is the most common pattern for “that/the …”:
- rumah besar itu = that big house / the big house
- jalan kecil itu = that small road / the small road

You can say itu jalan kecil, but then itu is more clearly the subject “that” and jalan kecil is like a description:

Itu jalan kecil. = That is a small road.

So:

jalan kecil itu = that/the small road (a noun phrase)
itu jalan kecil = that is a small road (a full sentence)

Why is there no word for “is” in jalan kecil itu berbahaya?

Indonesian normally does not use a separate verb for “to be” in sentences like this.

jalan kecil itu berbahaya literally: that small road dangerous
→ English needs “is”: that small road *is dangerous*

When the predicate is an adjective (like berbahaya, besar, kecil, panjang), Indonesian just puts it after the subject without “is”.

Examples:

Dia pintar. = He/She is smart.
Makanan ini enak. = This food is delicious.

What kind of word is berbahaya, and why not just bahaya?

bahaya is a noun meaning danger.
berbahaya is an adjective meaning dangerous.

The prefix ber- often turns a noun into an adjective or verb with the idea of “having / being in a state of”:

bahaya (danger) → berbahaya (dangerous / having danger)

In this sentence, we need an adjective (dangerous), so berbahaya is the natural choice.
jalan kecil itu bahaya can be heard in casual speech, but it sounds informal and a bit “slangy”; berbahaya is standard.

Does kecil here mean “small” or “narrow”?

Literally, kecil means small, but when talking about roads, jalan kecil is usually understood as a small, narrow road, often a side street or alley.

You could also say:

jalan sempit = narrow road (focusing specifically on width)

So:

jalan kecil = small road (implies narrow, minor road)
jalan sempit = narrow road (emphasis on the narrowness)

Both can be dangerous in the rain; kecil sounds a bit more neutral and common.

What is the function of saat in saat hujan deras? Can I replace it with ketika or waktu?

saat here means when / at the time (that) or during.

berbahaya saat hujan deras ≈ is dangerous when it’s raining heavily / during heavy rain

You can replace saat with:

ketika – very common for when in past narratives or general statements
waktu – literally time, but also used as when (more informal)

So, these are all possible with little or no change in meaning:

Jalan kecil itu berbahaya saat hujan deras.
Jalan kecil itu berbahaya ketika hujan deras.
Jalan kecil itu berbahaya waktu hujan deras. (more conversational)

Is hujan deras a complete phrase meaning “heavy rain”, or should there be a word for “is” or “very”?

hujan deras is a normal noun + adjective phrase:

hujan = rain
deras = heavy, intense (for rain or water flow)

So hujan deras = heavy rain. No extra word is needed.

Some related patterns:

angin kencang = strong wind
salju lebat = heavy snow (in some contexts)

You could add intensifiers if you want:

hujan sangat deras = very heavy rain
hujan deras sekali = very heavy rain

Could I say pada saat hujan deras instead of saat hujan deras?

Yes.

pada saat hujan deras = at the time of heavy rain / when it’s raining heavily

pada is a general preposition (at, on, in), so pada saat literally means “at the time (when)”. It sounds a bit more formal or explicit than just saat, but both are correct:

Jalan kecil itu berbahaya saat hujan deras.
Jalan kecil itu berbahaya pada saat hujan deras. (slightly more formal)

Can I change the word order and say Saat hujan deras, jalan kecil itu berbahaya?

Yes, that is perfectly natural.

Indonesian often puts the time clause at the beginning:

Saat hujan deras, jalan kecil itu berbahaya.
= When there is heavy rain, that small road is dangerous.

Both orders are correct:

Jalan kecil itu berbahaya saat hujan deras.
Saat hujan deras, jalan kecil itu berbahaya.

The meaning is the same; the second just emphasizes the time condition first.

Why is it hujan deras, not deras hujan? What is the usual adjective order?

In Indonesian, adjectives usually come after the noun they describe:

jalan kecil = small road
rumah besar = big house
hujan deras = heavy rain

So the pattern is: noun + adjective.
deras hujan would sound wrong in standard Indonesian.

Is itu here more like “that” (pointing) or like “the” (definite article)?

It can do both jobs, depending on context.

In jalan kecil itu, itu can mean:

that small road (specific, maybe visible or mentioned before)
the small road (a particular road both speaker and listener know about)

Indonesian does not have separate words for “the” and “that”. itu often covers both that and the, especially when used after a noun phrase. Context tells you whether it feels more like that or the in English.