Learn Turkish: Odaya koymamız gereken çıngırak kırmızı değil. - The rattle we must put in the room is not red.

This is the relative clause of necessity taught in this lesson. We take the verb koymak (to put), turn it into a verbal noun with -ma, and add the possessive -mız to show who must do it ("our putting"). Then we add gereken ("that is necessary"). Literally, it means "the rattle that our putting [it] is necessary", which smoothly translates to "the rattle we must put".

The English translation says "in the room", so why is it Odaya (to the room) instead of Odada (in the room)?

The verb koymak (to put) requires the dative case (-a/-e) because it focuses on the destination of the movement. You are putting the rattle into the room. While English naturally says "put in", Turkish grammar treats "putting" as a directional action, so we must use Odaya.

Why can't we just use koymalıyız here since it means "we must put"?

Koymalıyız is a conjugated verb meant to be the main predicate of a sentence, as in Çıngırağı odaya koymalıyız (We must put the rattle in the room). However, in this sentence, "we must put" is describing a noun (çıngırak). To modify a noun with a necessity, you have to use a relative clause like koymamız gereken (acting essentially as an adjective: "the we-must-put rattle").

What is the main subject of the Turkish sentence?