Learn Hardcore Indonesian: Kamar kos saya tenang di malam hari.

Indonesian normally does not use a verb like “to be” (am/is/are) before adjectives.
The adjective itself (tenang) functions as the predicate, so kamar kos saya tenang literally feels like “my boarding room quiet” but means “my boarding room is quiet.”
You could add adalah in some contexts, but Kamar kos saya adalah tenang is unnatural; with adjectives you almost always leave adalah out.

Is tenang an adjective or a verb here?

In this sentence tenang is an adjective meaning “quiet, calm, peaceful.”
Indonesian adjectives can often be used like stative verbs (describing a state), but grammatically here it’s functioning as the predicate adjective describing kamar kos saya.
So kamar kos saya tenang ≈ “my boarding room is quiet/calm.”

What exactly does kamar kos mean, and how is it different from just kamar or kos?

kamar = room (any room: bedroom, hotel room, etc.)
kos (also written kost) = a boarding house / rented room in a boarding house
kamar kos = the room you rent in a boarding house, i.e. your boarding room or rented room

So kamar kos saya is more specific than kamar saya; it tells you what kind of room it is (a rented boarding-room).

Why is saya at the end (kamar kos saya) instead of before the noun like in English?

In Indonesian, possessive pronouns usually come after the noun they modify:

kamar saya = my room
buku saya = my book
kamar kos saya = my boarding room

You can also attach -ku to the noun (more informal):

kamarku, kamar kosku = my room / my boarding room

Putting saya after the noun phrase is the normal, neutral way to show possession.

Could I just say kos saya instead of kamar kos saya?

Yes, in many contexts people say kos saya and it’s understood to mean the place where you stay (your rented room/boarding house).
kamar kos saya explicitly focuses on the room itself, while kos saya can refer to the place more generally (the boarding house, your lodging).
Both can work, depending on whether you want to emphasise the room or just your place of stay.

What does di malam hari literally mean, and why both malam and hari?

Literally, di malam hari is “in the night time”:

di = in/at
malam = night
hari = day/time

malam hari is a set phrase meaning “nighttime / at night”.
Using both malam and hari makes it sound a bit more complete or formal than just malam, but the meaning is essentially “at night.”

Could I say just di malam or just malam hari without di?

Yes:

di malam hari – common, sounds natural
malam hari – also fine; in many sentences you can drop di with time expressions
malam alone – also possible in casual speech

Example alternatives:

Kamar kos saya tenang malam hari.
Kamar kos saya tenang malam.

All still mean that the room is quiet at night, though di malam hari is a very typical, clear form.

Should I use di or pada with time expressions like malam hari?

Both can be used, but they differ in style:

di malam hari – very common in everyday speech and writing
pada malam hari – more formal or written style (reports, news, essays)

For everyday conversation, di malam hari or just malam hari is usually preferred.

Is the word order fixed, or can I move di malam hari to the front?

You can move the time phrase:

Kamar kos saya tenang di malam hari.
Di malam hari, kamar kos saya tenang.

Both are grammatical and natural.
Putting Di malam hari at the beginning slightly emphasizes the time (focusing on at night as the setting).

How would I say “My boarding room was quiet last night” or “will be quiet tonight” in Indonesian?

Indonesian usually uses time words, not verb conjugation, to show tense:

Kamar kos saya tenang tadi malam.
→ My boarding room was quiet last night.
Kamar kos saya akan tenang malam ini.
→ My boarding room will be quiet tonight.

The adjective tenang stays the same; you just add time words (tadi malam, malam ini) and optionally akan for future.

Is tenang the most common word for “quiet” when talking about a place, or are there other options?

tenang means quiet in the sense of calm, peaceful, not noisy.
Two other common words are:

sepi – quiet because not many people are there, deserted, not busy
sunyi – very quiet, silent, often with a slightly poetic or lonely feel

So:

Kamar kos saya tenang di malam hari. – calm, peaceful at night.
Kamar kos saya sepi di malam hari. – not many people around / feels deserted at night.