Learn Hardcore Indonesian: Bawang merah wangi ketika segar.

In Indonesian, bawang merah usually means “shallots.” For the large bulb onions you’d say bawang bombai (often spelled “bombay” in everyday writing); for the red variety, bawang bombai merah. Related terms:

bawang putih = garlic
daun bawang = scallions/spring onions (context-dependent, but commonly “green onions”)

Why is there no “to be” verb before wangi?

Indonesian doesn’t need a copula with adjectives. Structure is simply: noun + adjective = “X is [adj].”

Natural: Bawang merah wangi.
Don’t use adalah with adjectives: Bawang merah adalah wangi is ungrammatical/stilted. Use adalah before a noun phrase (e.g., Dia adalah guru).

Is this sentence generic or about specific onions?

It’s generic by default: “Shallots are fragrant when fresh.” To make it specific:

Bawang merah itu wangi ketika segar. = Those/that shallot is fragrant when fresh.
Bawang merahnya wangi. = The (aforementioned) shallots smell good.
Singular, specific: Satu siung/butir bawang merah ini wangi.

Can I say “Fresh shallots are fragrant” using yang?

Yes:

Bawang merah yang segar wangi.
Bawang merah yang segar itu wangi.
These treat “fresh” as part of the noun phrase (“fresh shallots”). Your original with ketika segar reads more like “whenever they’re fresh.” Both are natural.

What’s the difference between ketika, saat, waktu, and kalau here?

All can mean “when,” with nuance:

ketika segar = when fresh (neutral to slightly formal).
saat segar = at the time it’s fresh (very common).
waktu is more colloquial and usually takes a fuller clause: waktu (bawang merah masih) segar or waktu masih segar.
kalau segar = if/when it’s fresh (conditional/habitual; very conversational).

Where did the subject go in ketika segar? Should it be ketika bawang merah segar?

The subject is understood to be the same as the main clause, so it’s omitted. Both are grammatical:

Bawang merah wangi ketika segar.
Bawang merah wangi ketika bawang merah (itu) segar. (clear but repetitive; usually avoided) Adding masih is common: ketika masih segar = when still fresh.

What does wangi mean exactly? How does it differ from harum or bau?

wangi = pleasantly fragrant (neutral–positive; very common).
harum = pleasantly fragrant, often a bit literary/elevated (flowers, perfume).
bau = smell/odor; often negative unless specified.
berbau = has a smell; in everyday speech it tends to sound negative unless you add a positive word (e.g., berbau harum).
aroma/beraroma = aroma/have an aroma (neutral to positive; slightly formal).
Examples: Baunya wangi. / Aromanya kuat.

Can I use a verb like “smell” instead of an adjective?

Yes:

Bawang merah beraroma kuat ketika segar.
Wangi bawang merah lebih tercium ketika segar. Avoid berwangi (not idiomatic). Berbau is possible but can imply an unpleasant smell unless clarified (e.g., berbau harum/wangi).

Is the word order fixed? Can I put the time clause first?

You can front it:

Ketika segar, bawang merah wangi. The comma is optional but helpful.

How do you pronounce the words?

Approximation + IPA:

bawang: bah-wahng [ba.waŋ] (final ng as in “sing”)
merah: muh-RAH [mə.rah] (tap/trill the r; h pronounced)
wangi: WAH-ngi [wa.ŋi] (ng as in “sing” + ee)
ketika: kuh-TEE-kah [kə.ˈti.ka]
segar: suh-GAR [sə.ˈgar] (hard g; tapped/trilled r) Vowels are pure; stress is light, usually penultimate.

Does segar only mean “fresh” for food?

Mainly yes (not stale/spoiled). Related uses:

(masih) segar = still fresh (food).
baru dipetik/dipotong = freshly picked/cut.
segar bugar = refreshed/fit (people).
In your sentence, it’s the food sense.

How do I express plurals or count pieces?

Indonesian leaves plural unmarked unless needed. To count/specify:

dua siung bawang merah = two pieces/cloves of shallot (very common in recipes).
dua butir bawang merah = two bulbs (also heard).
banyak bawang merah = many shallots. Demonstratives make it specific: bawang merah ini/itu.

Is bawang merah a fixed term, or is merah just a color adjective here?

It’s both: literally “red onion,” but as a compound bawang merah is the standard term for shallots in Indonesian. Likewise:

bawang putih = garlic (literally “white onion”).
bawang bombai = bulb onion.

How would I ask “Do shallots smell good when fresh?”

Apakah bawang merah wangi ketika segar? (neutral/formal)
Bawang merah wangi kalau (masih) segar, ya? (colloquial check)
Bawang merah wangi ketika segar, kan? (tag question)

Does Indonesian mark tense here, or could it be past/future?

There’s no tense marking. The sentence states a general fact. Time comes from context or adverbs:

Past: Kemarin bawang merah ini wangi ketika (masih) segar.
Future: Nanti bawang merah itu wangi kalau masih segar.