Saat hujan deras, kereta bergerak pelan-pelan.

In this sentence, saat functions as “when,” introducing a time clause.

• ketika also means “when” and is largely interchangeable with saat, though ketika can sound slightly more formal or literary.
• waktu is a noun meaning “time,” so waktu hujan deras would literally be “the time of heavy rain,” not a conjunction.

Use saat or ketika to link events in time; reserve waktu when you talk about “time” itself.

Indonesian follows Noun + Adjective order, unlike English:

• hujan deras = “rain heavy” → heavy rain.
  Here, deras means “heavy” (specifically for water falling fast). You’ll also see hujan ringan (“light rain”) or hujan lebat (“torrential rain”).

• bergerak = “to move” (general motion).
• jalan by itself is “walk” (verb) or “road” (noun).
• berjalan usually means “to walk” (for people/animals), though it can mean “run/operate” for machines (e.g., mesin ini berjalan = “this machine runs”).
• berjalan lambat tends to evoke “walking slowly,” not a vehicle moving slowly.

So for vehicles you normally say kereta bergerak pelan-pelan to mean “the train moves slowly.”

The hyphens mark reduplication, which in Indonesian often turns an adjective into an adverb or softens its intensity.

• pelan = “slow” (adjective).
• pelan-pelan = “slowly,” suggesting a gradual, moderate pace.

Reduplication here makes it clear you’re describing how the action happens.

Indonesian has no definite or indefinite articles. Context tells you whether you mean “a train,” “the train,” or just “trains” in general.

• To specify, you can add demonstratives: kereta itu (“that train”) or kereta ini (“this train”).

Yes, both orders are correct and mean the same:

• Saat hujan deras, kereta bergerak pelan-pelan.
• Kereta bergerak pelan-pelan saat hujan deras.

The first version foregrounds the time condition; the second foregrounds the train’s action. Otherwise, no change in meaning.

Standard Indonesian uses the hyphen in reduplicated words: pelan-pelan.

• Writing pelan pelan without a hyphen is common in informal notes but not orthodox.
• Using just pelan can still be understood (“slow”), but it’s less clearly an adverb describing how the train moves.