Learn Turkish: Bebek üşümesin diye değiştirmemiz gereken kılıf bu. - This is the cover we must change so that the baby doesn't get cold.

This uses the relative clause of necessity taught in this lesson. You take the verb root (değiştir-), add the verbal noun suffix (-me), and attach the possessive ending for whoever needs to do the action (-miz for "our"). You then add gereken ("that is necessary") and the noun (kılıf). Literally, it translates to: "The cover that our changing is necessary."

Why is üşümesin used with diye here? Isn't üşümesin a command meaning "let him not get cold"?

Exactly! In Turkish, a common way to express purpose or intention ("so that...") is to use the imperative or optative mood followed by diye. The phrase Bebek üşümesin diye literally translates as "saying 'let the baby not get cold'," which functions perfectly in Turkish as "so that the baby doesn't get cold."

Why is bu at the very end of the sentence instead of the beginning?

You certainly could say Bu, bebek üşümesin diye değiştirmemiz gereken kılıf ("This is the cover..."). However, putting bu at the end changes the flow and emphasis by making it the predicate. The literal sentence structure becomes: "[The cover we must change...] is this one." It is a very natural way to single a specific object out.

What specific type of cover is a kılıf?