Kamar kos saya tenang di malam hari.

to be

Breakdown of Kamar kos saya tenang di malam hari.

• di: at
• malam hari: the night
• tenang: calm

Questions & Answers about Kamar kos saya tenang di malam hari.

Why is there no word for “is” in 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).