Learn Turkish: Gece üşürsün diye termosu evde bırakmamalısın. - You must not leave the thermos at home in case you get cold at night.

Literally, it means 'saying/thinking you will get cold'. Using the aorist (üşürsün) plus diye shows a precautionary thought. You are taking an action (bringing the thermos) because you anticipate a possibility (getting cold). This is the standard way to express 'in case' in Turkish.

Could we use the conditional here, like üşürsen diye?

Yes! Üşürsen diye (if you get cold + diye) is also very common for 'in case'. The aorist (üşürsün diye) focuses slightly more on the expectation ('thinking you will get cold'), while the conditional (üşürsen diye) focuses on the condition ('thinking about what happens if you get cold'). Both work perfectly for 'in case'.

Why don't we use a negative verb here, like 'so you don't get cold'?

You could use a negative subjunctive like üşümeyesin diye, but that shifts the meaning to 'so that you don't get cold' (an active goal). The positive aorist üşürsün diye specifically expresses preparing for a possible event ('in case you do get cold').

Let's break down bırakmamalısın. Why are there two 'ma' syllables?

The first -ma is the regular negation suffix, and the second is part of the necessity suffix -malı (must / should). Together, bırak-ma-malı-sın means 'leave-not-must-you', which naturally translates to 'you must not leave'.