Learn Hardcore Indonesian: Air hangat ini bisa diminum setelah makan malam. - This warm water can be drunk after dinner.

In Indonesian, air means water, not the gas we breathe. This is just a false friend between English and Indonesian. For air as in English, Indonesian uses udara. So air hangat literally means warm water.

What exactly does hangat mean? Is it "warm" or "hot"? How is it different from panas?

Hangat usually means warm (pleasantly warm, not too hot).
Panas means hot (high temperature, can burn or be uncomfortable).
So air hangat is like warm water or lukewarm water, while air panas suggests very hot or boiling water.

Why is it air hangat ini and not ini air hangat? Are both possible?

Both are grammatically correct, but they sound a bit different.

air hangat ini = this warm water (demonstrative after the noun, the most common pattern in Indonesian: noun + modifier + ini/itu).
ini air hangat = this is warm water (often understood as an equational sentence: “this is warm water”).
In your sentence, air hangat ini is better because it clearly functions as the subject: This warm water can be drunk…

Why is ini used here instead of itu? What’s the difference between ini and itu?

Ini means this, referring to something close to the speaker (physically, or just introduced in the context).
Itu means that, usually something farther away or already known in the conversation.
So air hangat ini suggests water that is right here, or just prepared and being talked about now. Air hangat itu would refer to some other water, not as “immediate” to the speaker.

What does bisa mean here, and how is it different from boleh or dapat?

In this sentence, bisa means can / is able to / is possible to. It usually expresses ability or possibility.

bisa diminum = can be drunk / is drinkable.
boleh diminum = may be drunk / it’s allowed to drink it (focus on permission).
dapat diminum is similar to bisa diminum, a bit more formal or neutral, often used in writing.
Here bisa emphasizes that the warm water is suitable / possible to drink after dinner.

Why is it diminum and not just minum? What does the di- do?

Minum is the base verb to drink.
When you add di- as a prefix, it usually makes a passive form: diminum = to be drunk.
So:

minum air hangat ini = drink this warm water.
air hangat ini bisa diminum = this warm water can be drunk.
In your sentence, the focus is on the water (what can be done to it), so the passive form diminum is natural.

Who is actually doing the drinking? The sentence doesn’t mention any subject like "you" or "we". Is that normal?

Yes, that’s very normal in Indonesian. The passive bisa diminum here is impersonal: it means “can be drunk (by people / by you / by us)” without specifying who.
Indonesian often omits the agent if it’s obvious or not important.
If you want to specify, you could say:

Air hangat ini bisa kamu minum setelah makan malam.
Kita bisa minum air hangat ini setelah makan malam.

Could the sentence be active instead of passive? How would that look, and is the meaning different?

Yes, you can make it active:

Kamu bisa minum air hangat ini setelah makan malam.
Kita bisa minum air hangat ini setelah makan malam.
The basic meaning is the same (you/we can drink this warm water after dinner), but the focus shifts:
Passive air hangat ini bisa diminum focuses on the water (it is drinkable).
Active kamu/kita bisa minum air hangat ini focuses on who will drink it.

How is time expressed here? Does bisa diminum mean "can be drunk now" or "can be drunk later"?

Indonesian verbs usually do not mark tense (past/present/future) directly.
The time is indicated by context and time expressions like setelah makan malam.
Here, bisa diminum is neutral for time; setelah makan malam tells you when: it can be drunk after dinner (in the future relative to now, or as a general rule).

What does setelah mean exactly, and is it the same as sesudah or habis?

Setelah means after. In this sentence, setelah makan malam = after dinner.

Setelah and sesudah are almost interchangeable in everyday use; sesudah can sound slightly more formal or old-fashioned in some contexts, but both are fine.
Habis in colloquial speech can also mean after (literally “finished”): habis makan malam = after finishing dinner. It’s more informal.

Is makan malam a verb phrase (to eat dinner) or a noun (dinner) here?

It can function as both, depending on context.
In setelah makan malam, it can be understood either as:

“after (we) eat dinner” (verb phrase), or
“after dinner” (a nominalized activity).
Indonesian often doesn’t strictly separate these; makan malam naturally covers both “the act of eating dinner” and “the dinner event”.

Can setelah makan malam be moved to the beginning of the sentence?

Yes, that’s very natural:

Setelah makan malam, air hangat ini bisa diminum.
The meaning is the same. Putting the time expression first just emphasizes when more strongly. Indonesian word order is quite flexible for elements like time, place, and manner.

Does air hangat always mean plain warm water, or could it be a warm drink like tea?

Literally, air hangat is warm water, usually understood as plain water that has been warmed.
For other warm drinks, Indonesian tends to name the drink:

teh hangat = warm tea
susu hangat = warm milk
If you say air hangat in everyday conversation, people will normally think of plain warm water, not tea or coffee.